feat: add merge commit version strategy - #5140

Description

Adds a configurable MergeCommit version strategy for GitFlow and GitHubFlow main branches with prevent-increment.of-merged-branch enabled.

The strategy walks the target branch's first-parent history, uses the latest target-branch version tag as the reset boundary, and consolidates the highest effective increment across the post-tag history. Recognized merges contribute the source branch configuration, while direct target commits and merges whose source cannot be resolved contribute the target branch configuration. Repeated increments of the same kind do not accumulate. Source-branch commit-message increments and reset messages are honored, as is prevent-increment.when-branch-merged; nested merge branch configurations do not replace the directly merged branch configuration.

The strategy also preserves tags introduced by merged branches as alternative semantic versions and suppresses the regular tagged-commit candidate while active, so a larger target-branch increment cannot override the merged-branch increment. Each alternative tag tracks only increments encountered from the point where it enters the target history, allowing post-tag work to advance a higher merged tag without reapplying earlier target work. Historical source references are scored by graph distance before local-versus-remote preference is applied. The strategy is enabled in the GitFlow and GitHubFlow defaults and can be disabled through strategies.

Related Issue

Resolves #4433

Motivation and Context

Outside the mainline strategy, a no-fast-forward merge currently falls back to the target branch's configured increment. That makes a patch hotfix merged into a minor-increment main branch produce a minor version, and a minor feature merged into a patch-increment main branch produce a patch version. This change lets the merged branch determine the increment while retaining the existing tag-based version source and non-cumulative workflow semantics.

How Has This Been Tested?

Tested on Linux with .NET SDK 10.0.400:

• Added 35 integration cases covering GitFlow and GitHubFlow acceptance scenarios, disabled strategy behavior, repeated merges, highest-increment selection, tag reset boundaries, commit-message increments and resets, when-branch-merged, nested merges, merged-branch tags and their post-tag increments (including a tag at the source tip with no later work), unrelated histories, inherited source-branch increments (including ignored commits, reused local names, valid remote refs, sources that later absorb the topic, and retained branches that later absorb the target), inherited main-branch configurations, target commits before and after recognized merges, unrecognized later merges with tags, older higher target tags behind later lower tags, merge-message ordering, and timestamp-skewed tags and first-parent traversal.
• Added a focused source-selection unit case covering same-named local and remote refs with different ancestry distances.
• dotnet build ./src/GitVersion.slnx --configuration Release --no-restore --verbosity minimal — succeeded with 0 warnings and 0 errors.
• dotnet test --solution ./src/GitVersion.slnx --configuration Release --no-build --output Normal — 37,238 succeeded, 0 failed.
• dotnet format ./src/GitVersion.slnx --verify-no-changes --no-restore --verbosity minimal — succeeded.

Screenshots (if appropriate):

N/A

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.