fix(writer): reject commits with unchanged trees

[KSHITIZ6341]

What

• Return ErrNothingToCommit when staged operations rebuild the same root tree as the parent commit.
• Add a regression test for updating an existing blob with identical content.

Why

PackfileWriter.HasObjects() reports whether blob or tree objects were staged, not whether the resulting repository tree changed. Re-staging identical content therefore rebuilt the same root tree but still allowed Commit() to create an empty commit, which could then produce a no-op receive-pack request.

Good catch by @jdstrand; the reproduction clearly isolated the distinction between staged packfile objects and an unchanged commit tree.

Fixes #378

How

After pending trees are built, Commit() compares the rebuilt root tree hash with the parent commit's tree hash before creating a commit object. A match returns the existing ErrNothingToCommit error. Commits whose final tree differs from the parent continue through the existing path unchanged.

The signer unit-test fixtures now use a distinct parent tree so they continue to represent a real changed-tree commit.

Remarks

Behavioral effect: callers now receive ErrNothingToCommit for semantically unchanged staged updates, including an identical UpdateBlob, instead of receiving a new commit. There is no public API change and changed-tree commits are unaffected.

Checks performed:

• go test -race -parallel 6 --short -count=1 across all packages except protocol/signing
• go test . -run '^TestStagedWriter_Commit_(UnchangedTree|SignerError|SignerInvoked)$' -count=1
• make lint
• make lint-staticcheck
• git diff --check

make test-unit could not complete locally because the host does not have the gpg and gpgsm executables required by protocol/signing; all other packages passed with the race detector. make test-integration could not start its Gitea fixture because no local Docker provider is available, so 0 of 321 integration specs ran locally.

Author Checklist

• Tests added/updated.
• Documentation is not required for this internal behavior correction.
• Changes have been tested locally.

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