NBS-on-Git: add GitBlobstore-backed NBS store and empty-remote bootstrap

[coffeegoddd]

Add nbs.NewGitStore wiring plus fetch/push semantics and tests to open against an empty Git remote and bootstrap refs/dolt/data on first write.

[Copilot]

Pull request overview

This PR adds support for opening NBS stores against empty Git remotes and bootstrapping the refs/dolt/data reference on the first write. It provides a new NewNoConjoinGitStore function and implements fetch/push semantics that gracefully handle missing remote references.

Changes:

• Added NewNoConjoinGitStore function to create GitBlobstore-backed NBS stores without conjoin support
• Implemented empty-remote bootstrap logic: fetch operations treat missing remote refs as empty stores, and push operations create refs that don't exist
• Enhanced error handling to distinguish between missing remote refs and other fetch failures

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
go/store/nbs/store.go	Added NewNoConjoinGitStore factory function following established patterns
go/store/nbs/git_blobstore_empty_remote_test.go	Comprehensive tests for empty remote handling at the NBS layer
go/store/blobstore/internal/git/impl.go	Enhanced FetchRef to detect missing remote refs and removed PushRefWithLease validation to allow bootstrap pushes
go/store/blobstore/internal/git/impl_test.go	Tests for missing remote ref detection and bootstrap push semantics
go/store/blobstore/internal/git/api.go	Updated PushRefWithLease documentation to clarify bootstrap semantics
go/store/blobstore/git_blobstore.go	Implemented empty remote handling in syncForRead and remoteManagedWrite
go/store/blobstore/git_blobstore_test.go	Added test verifying Put operation bootstraps empty remotes

---

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[coffeegoddd]

@coffeegoddd DOLT

read_tests	from_latency	to_latency	percent_change
covering_index_scan	0.54	0.55	1.85
groupby_scan	10.09	9.91	-1.78
index_join	1.82	1.82	0.0
index_join_scan	1.34	1.34	0.0
index_scan	21.89	22.28	1.78
oltp_point_select	0.27	0.27	0.0
oltp_read_only	5.28	5.28	0.0
select_random_points	0.54	0.54	0.0
select_random_ranges	0.55	0.55	0.0
table_scan	21.89	22.28	1.78
types_table_scan	66.84	66.84	0.0

write_tests	from_latency	to_latency	percent_change
oltp_delete_insert	6.32	6.32	0.0
oltp_insert	3.07	3.07	0.0
oltp_read_write	11.45	11.45	0.0
oltp_update_index	3.19	3.19	0.0
oltp_update_non_index	3.13	3.13	0.0
oltp_write_only	6.09	6.09	0.0
types_delete_insert	6.91	6.79	-1.74

[coffeegoddd]

@coffeegoddd DOLT

read_tests	from_latency	to_latency	percent_change
covering_index_scan	0.54	0.54	0.0
groupby_scan	9.91	10.09	1.82
index_join	1.82	1.82	0.0
index_join_scan	1.34	1.34	0.0
index_scan	21.89	21.89	0.0
oltp_point_select	0.27	0.27	0.0
oltp_read_only	5.28	5.28	0.0
select_random_points	0.54	0.54	0.0
select_random_ranges	0.55	0.55	0.0
table_scan	21.89	22.28	1.78
types_table_scan	65.65	66.84	1.81

write_tests	from_latency	to_latency	percent_change
oltp_delete_insert	6.32	6.32	0.0
oltp_insert	3.02	3.07	1.66
oltp_read_write	11.45	11.45	0.0
oltp_update_index	3.25	3.19	-1.85
oltp_update_non_index	3.13	3.13	0.0
oltp_write_only	6.09	6.09	0.0
types_delete_insert	6.79	6.79	0.0

[coffeegoddd]

@coffeegoddd DOLT

test_name	from_latency_p95	to_latency_p95	percent_change
tpcc-scale-factor-1	61.08	61.08	0.0

test_name	from_server_name	from_server_version	from_tps	to_server_name	to_server_version	to_tps	percent_change
tpcc-scale-factor-1	dolt	94fa063	37.97	dolt	5bee09c	37.55	-1.11

[coffeegoddd]

@coffeegoddd DOLT

read_tests	from_latency	to_latency	percent_change
covering_index_scan	0.55	0.55	0.0
groupby_scan	10.09	9.91	-1.78
index_join	1.82	1.86	2.2
index_join_scan	1.34	1.34	0.0
index_scan	21.89	21.89	0.0
oltp_point_select	0.28	0.27	-3.57
oltp_read_only	5.37	5.37	0.0
select_random_points	0.54	0.54	0.0
select_random_ranges	0.56	0.55	-1.79
table_scan	22.28	22.28	0.0
types_table_scan	65.65	66.84	1.81

write_tests	from_latency	to_latency	percent_change
oltp_delete_insert	6.32	6.43	1.74
oltp_insert	3.07	3.07	0.0
oltp_read_write	11.65	11.45	-1.72
oltp_update_index	3.25	3.25	0.0
oltp_update_non_index	3.13	3.13	0.0
oltp_write_only	6.09	6.09	0.0
types_delete_insert	6.91	6.91	0.0

[coffeegoddd]

@coffeegoddd DOLT

test_name	from_latency_p95	to_latency_p95	percent_change
tpcc-scale-factor-1	62.19	62.19	0.0

test_name	from_server_name	from_server_version	from_tps	to_server_name	to_server_version	to_tps	percent_change
tpcc-scale-factor-1	dolt	94fa063	37.0	dolt	53fa46c	36.97	-0.08

[coffeegoddd]

@coffeegoddd DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
964d718	ok	5937471

version	total_tests
964d718	5937471

correctness_percentage
100.0

[coffeegoddd]

@coffeegoddd DOLT

test_name	from_latency_p95	to_latency_p95	percent_change
tpcc-scale-factor-1	61.08	61.08	0.0

test_name	from_server_name	from_server_version	from_tps	to_server_name	to_server_version	to_tps	percent_change
tpcc-scale-factor-1	dolt	94fa063	37.63	dolt	964d718	37.97	0.9

[github-actions]

@coffeegoddd DOLT

name	add_cnt	delete_cnt	update_cnt	latency
adds_only	60000	0	0	0.6
adds_updates_deletes	60000	60000	60000	3.04
deletes_only	0	60000	0	1.43
updates_only	0	0	60000	1.93

[github-actions]

@coffeegoddd DOLT

test_name	detail	row_cnt	sorted	mysql_time	sql_mult	cli_mult
batching	LOAD DATA	10000	1	0.05	2
batching	batch sql	10000	1	0.07	1.86
batching	by line sql	10000	1	0.07	2
blob	1 blob	200000	1	0.91	4.18	4.34
blob	2 blobs	200000	1	0.86	4.8	4.65
blob	no blob	200000	1	0.87	2.86	2.76
col type	datetime	200000	1	0.83	2.61	2.59
col type	varchar	200000	1	0.68	3.74	3.59
config width	2 cols	200000	1	0.79	2.92	2.54
config width	32 cols	200000	1	1.93	3.08	2.76
config width	8 cols	200000	1	0.95	2.87	3.47
pk type	float	200000	1	0.87	2.46	2.39
pk type	int	200000	1	0.81	2.59	2.48
pk type	varchar	200000	1	1.59	1.65	1.52
row count	1.6mm	1600000	1	5.88	2.9	2.81
row count	400k	400000	1	1.44	2.92	2.81
row count	800k	800000	1	2.81	3.02	2.88
secondary index	four index	200000	1	3.81	1.26	1.08
secondary index	no secondary	200000	1	0.88	2.81	2.74
secondary index	one index	200000	1	1.13	2.58	2.32
secondary index	two index	200000	1	2.06	1.74	1.67
sorting	shuffled 1mm	1000000	0	5.83	2.53	2.34
sorting	sorted 1mm	1000000	1	5.87	2.51	2.42

[github-actions]

@coffeegoddd DOLT

name	detail	mean_mult
dolt_blame_basic	system table	1.25
dolt_blame_commit_filter	system table	1.16
dolt_commit_ancestors_commit_filter	system table	0.64
dolt_commits_commit_filter	system table	1.1
dolt_diff_log_join_from_commit	system table	3.07
dolt_diff_log_join_to_commit	system table	3.11
dolt_diff_table_from_commit_filter	system table	1.23
dolt_diff_table_to_commit_filter	system table	1.24
dolt_diffs_commit_filter	system table	1.06
dolt_history_commit_filter	system table	1.52
dolt_log_commit_filter	system table	1.1