cmd/git(fix[set_head]): Make set_head doctest git-version-agnostic

why: git 2.54 changed `git remote set-head` output: when origin/HEAD
already points at the branch it prints "'origin/HEAD' is unchanged and
points to 'master'" instead of "origin/HEAD set to master". The doctest
asserted the older literal string, so it failed under the git on CI.
The failure is independent of the pytest 9.1 fixture changes -- it was
latent until the plugin could import and the suite actually ran.

what:
- Assert version-stable invariants instead of the literal message:
  `'master' in set_head(auto=True)` and that `set_head('master')`
  returns a str

Author: tony

src/libvcs/cmd/git.py

@@ -4094,17 +4094,19 @@ def set_head(
 
         Examples
         --------
-        >>> GitRemoteCmd(
+        >>> remote = GitRemoteCmd(
         ...     path=example_git_repo.path,
         ...     remote_name='origin'
-        ... ).set_head(auto=True)
-        'origin/HEAD set to master'
+        ... )
 
-        >>> GitRemoteCmd(
-        ...     path=example_git_repo.path,
-        ...     remote_name='origin'
-        ... ).set_head('master')
-        ''
+        The exact message wording varies across git versions (git 2.54+ reports
+        "is unchanged" when HEAD already points at the branch), so assert on the
+        stable parts rather than the literal string:
+
+        >>> 'master' in remote.set_head(auto=True)
+        True
+        >>> isinstance(remote.set_head('master'), str)
+        True
         """
         local_flags: list[str] = [self.remote_name]