GitBlobstore: implement Put with CAS retries + configurable identity; add Put tests

[coffeegoddd]

This PR adds the first GitBlobstore write path: GitBlobstore.Put, implemented on top of the existing internal/git.GitAPI plumbing. It also adds unit tests for Put, including a contention scenario to verify we don’t clobber concurrent writers.

[Copilot]

Pull request overview

This PR implements the Put method for GitBlobstore, enabling write operations with compare-and-swap (CAS) retry logic to handle concurrent writers. The implementation uses exponential backoff and preserves other keys when conflicts occur, making it safe for multi-writer scenarios.

Changes:

• Implemented GitBlobstore.Put with CAS retry logic and exponential backoff for handling concurrent updates
• Added configurable identity support via NewGitBlobstoreWithIdentity for controlling git commit author/committer
• Added comprehensive unit tests for Put including round-trip verification, overwrite behavior, and contention scenarios

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
go/store/blobstore/git_blobstore.go	Implements Put method with CAS retries, adds identity configuration, and includes helper functions for building commits and managing temporary indexes
go/store/blobstore/git_blobstore_test.go	Adds test infrastructure (requireGitOnPath, testIdentity) and comprehensive tests for Put including basic round-trip, overwrite, and concurrent writer scenarios

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[coffeegoddd]

@coffeegoddd DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
18dea9b	ok	5937471

version	total_tests
18dea9b	5937471

correctness_percentage
100.0

[reltuk]

Looks good. A couple comments.

[reltuk]

Can you do some testing and verify whether this is idempotent? If it isn't, can we make it so?

That is to say: If the file already exists, we should succeed here (AFAIK...)

[coffeegoddd]

I believe its idempotent for files (overwrites) but will error if the a directory prefix is already staged, then you try writing that. Added a test that demonstrates this, though I don't expect this to be a problem for us since we are writting files only: https://github.com/dolthub/dolt/pull/10417/changes#diff-7e67da3161a3fc914b18c9152a5ea340fbf582ca9cb2efc2dc5ef6ab7b0d5c20R335

[coffeegoddd]

@coffeegoddd DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
f8fd7ee	ok	5937471

version	total_tests
f8fd7ee	5937471

correctness_percentage
100.0

[github-actions]

@coffeegoddd DOLT

name	add_cnt	delete_cnt	update_cnt	latency
adds_only	60000	0	0	0.62
adds_updates_deletes	60000	60000	60000	3.09
deletes_only	0	60000	0	1.48
updates_only	0	0	60000	1.97

[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	0.08	1.63
batching	by line sql	10000	1	0.09	1.44
blob	1 blob	200000	1	0.89	4.3	4.51
blob	2 blobs	200000	1	0.89	4.75	4.56
blob	no blob	200000	1	0.9	2.79	2.69
col type	datetime	200000	1	0.83	2.63	2.57
col type	varchar	200000	1	0.67	3.85	3.85
config width	2 cols	200000	1	0.81	3.02	2.41
config width	32 cols	200000	1	1.93	3.14	2.76
config width	8 cols	200000	1	0.95	2.92	2.72
pk type	float	200000	1	0.95	2.29	2.19
pk type	int	200000	1	0.79	2.67	2.54
pk type	varchar	200000	1	1.56	1.7	1.59
row count	1.6mm	1600000	1	5.72	3.01	2.77
row count	400k	400000	1	1.45	2.93	2.79
row count	800k	800000	1	2.86	2.99	2.75
secondary index	four index	200000	1	3.66	1.34	1.07
secondary index	no secondary	200000	1	0.89	2.81	2.71
secondary index	one index	200000	1	1.14	2.59	2.23
secondary index	two index	200000	1	2	1.82	1.72
sorting	shuffled 1mm	1000000	0	5.41	2.76	2.5
sorting	sorted 1mm	1000000	1	5.39	2.77	2.61

[github-actions]

@coffeegoddd DOLT

name	detail	mean_mult
dolt_blame_basic	system table	1.23
dolt_blame_commit_filter	system table	1.15
dolt_commit_ancestors_commit_filter	system table	0.63
dolt_commits_commit_filter	system table	1.11
dolt_diff_log_join_from_commit	system table	2.95
dolt_diff_log_join_to_commit	system table	3
dolt_diff_table_from_commit_filter	system table	1.24
dolt_diff_table_to_commit_filter	system table	1.32
dolt_diffs_commit_filter	system table	1.09
dolt_history_commit_filter	system table	1.46
dolt_log_commit_filter	system table	1.05