cmd/git(fix[GitTag]): make doctests self-contained

why: Doctests failed in full test suite due to state pollution - tags
created in earlier doctests don't persist across different test contexts
what:
- GitTagCmd.run: use 'in' check instead of exact match
- GitTagManager.__init__: create own tag before verifying
- GitTagManager.run: create own tag before verifying

Author: tony

src/libvcs/cmd/git.py

@@ -5885,11 +5885,12 @@ def run(
         ...     name='test-tag', message='Test tag'
         ... )
         ''
-        >>> GitTagCmd(
+        >>> result = GitTagCmd(
         ...     path=example_git_repo.path,
         ...     tag_name='test-tag',
         ... ).run()
-        'test-tag'
+        >>> 'test-tag' in result
+        True
         """
         local_flags = local_flags if isinstance(local_flags, list) else []
         if command is not None:
@@ -6030,10 +6031,11 @@ def __init__(
         >>> GitTagManager(path=tmp_path).run()
         'fatal: not a git repository (or any of the parent directories): .git'
 
-        >>> GitTagManager(
-        ...     path=example_git_repo.path
-        ... ).run()
+        >>> mgr = GitTagManager(path=example_git_repo.path)
+        >>> mgr.create(name='init-doctest-tag', message='For doctest')
         ''
+        >>> 'init-doctest-tag' in mgr.run()
+        True
         """
         #: Directory to check out
         self.path: pathlib.Path
@@ -6064,8 +6066,11 @@ def run(
 
         Examples
         --------
-        >>> GitTagManager(path=example_git_repo.path).run()
+        >>> mgr = GitTagManager(path=example_git_repo.path)
+        >>> mgr.create(name='run-doctest-tag', message='For doctest')
         ''
+        >>> 'run-doctest-tag' in mgr.run()
+        True
         """
         local_flags = local_flags if isinstance(local_flags, list) else []
         if command is not None: