Stabilize COW Git snapshots

[adamziel]

What it does

Makes COW Git repository sync skip unchanged branch exports before writing a new snapshot commit. The adapter now compares the current Git tip tree with the proposed wordpress/ export plus generated database.sql.

Rationale

forkpress pull and agent worktrees depend on stable remote refs. If a clone/fetch path advances refs without a real WordPress file or database snapshot change, agents get noisy rebases and dirty-looking history.

Implementation

Adds cow_git_commit_matches_updates() and reuses a shared cow_git_blob_hash() helper. When the exported tree already matches the current commit, cow_git_sync_repository() skips the commit path entirely.

Testing instructions

php -d "extension=/home/claude/forkpress/ext/branchfs.so" tests/test_cow_git_server.php
make test-all
FORKPRESS_RUNTIME_BUNDLE=/dev/null cargo test -p forkpress
git diff --check

The new test_cow_git_server.php coverage asserts unchanged file/database snapshots keep the same ref, while real file and SQLite changes advance it.

[JanJakes]

Reviewed and recorded follow-up notes under the PR #16 section in issue #30: #30 (comment)