cmd/git(fix[stash.push]): Handle str paths in GitStashCmd.push

why: Plain str paths were silently ignored when building required_flags.
Only list and pathlib.Path were handled, so Git.stash.push(path="file.txt")
would not limit the stash to that path.
what:
- Change elif isinstance(path, pathlib.Path) to elif path is not None
- Remove unnecessary .absolute() conversion that broke relative paths
- Now handles str, Path, and list consistently

Author: tony

src/libvcs/cmd/git.py

@@ -4858,9 +4858,9 @@ def push(
         required_flags: list[str] = []
 
         if isinstance(path, list):
-            required_flags.extend(str(pathlib.Path(p).absolute()) for p in path)
-        elif isinstance(path, pathlib.Path):
-            required_flags.append(str(pathlib.Path(path).absolute()))
+            required_flags.extend(str(p) for p in path)
+        elif path is not None:
+            required_flags.append(str(path))
 
         if patch is True:
             local_flags.append("--patch")