Add dolt_preview_merge_conflicts table function

[tbantle22]

This table function shows which rows are conflicting for a table between two branches. Will error if there are schema conflicts

[reltuk]

Some quick comments on the implementation. I think I got enough of an overview to discuss further iteration...

[Copilot]

Pull Request Overview

This PR adds support for the new table function dolt_preview_merge_conflicts along with extensive tests and refactors conflict table schema handling. Key changes include:

• New test cases for dolt_preview_merge_conflicts and its error conditions in multiple query test files.
• Renaming and refactoring of functions (e.g. CalculateConflictSchema) with updated conflict row handling using new descriptors and offsets.
• Minor adjustments and improvements in downstream merge logic and functionality.

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
go/libraries/doltcore/sqle/enginetest/dolt_queries_schema_merge.go	Added test cases for preview merge conflicts with expected error strings.
go/libraries/doltcore/sqle/enginetest/dolt_queries_merge.go	Expanded test coverage for various preview merge conflicts scenarios.
go/libraries/doltcore/sqle/dtables/conflicts_tables_prolly.go	Refactored conflict schema calculation and row iteration using new value descriptors and offsets.
go/libraries/doltcore/sqle/dtablefunctions/init.go	Registered the new PreviewMergeConflictsTableFunction.
go/libraries/doltcore/sqle/dtablefunctions/dolt_preview_merge_conflicts_summary.go	Updated branch-to-root resolution to use a new rootInfo struct.
go/libraries/doltcore/dtablefunctions/dolt_diff.go	Corrected error message formatting.
go/libraries/doltcore/merge/*	Added helper method and streamlined merge logic with new getters.

Comments suppressed due to low confidence (3)

go/libraries/doltcore/sqle/dtables/conflicts_tables_prolly.go:56

• The function name has been updated to 'CalculateConflictSchema' (exported), so please verify that all callers across the codebase have been updated to reflect this naming change.

confSch, versionMappings, err := CalculateConflictSchema(baseSch, ourSch, theirSch)

go/libraries/doltcore/sqle/dtables/conflicts_tables_prolly.go:256

• [nitpick] Consider adding inline comments or expanding the function's documentation to explain how the various offset values are computed and used when populating conflict rows. This would improve readability and ease future maintenance.

func PutConflictRowVals(ctx *sql.Context, confVal ConflictVal, row sql.Row, offsets ConflictOffsets, vds ConflictValueDescriptors, ns tree.NodeStore) error {

go/libraries/doltcore/sqle/dtablefunctions/dolt_preview_merge_conflicts_summary.go:299

• [nitpick] Now that the function returns a 'rootInfo' struct with additional fields (rightCm and ancCm), it would be beneficial to enhance the documentation for this function, explaining the purpose of each field and the expected usage downstream.

func resolveBranchesToRoots(ctx *sql.Context, db dsess.SqlDatabase, leftBranch, rightBranch string) (rootInfo, error) {

[nicktobey]

Alright this looks good. I just have a couple points of confusion that I think can be cleared up with comments.

[coffeegoddd]

@tbantle22 DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
bbfbb8f	ok	5937457

version	total_tests
bbfbb8f	5937457

correctness_percentage
100.0

[coffeegoddd]

@coffeegoddd DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
f1f9dec	ok	5937457

version	total_tests
f1f9dec	5937457

correctness_percentage
100.0

[coffeegoddd]

@tbantle22 DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
8169350	ok	5937457

version	total_tests
8169350	5937457

correctness_percentage
100.0