sync/git(fix[update_repo]): Use quiet=True for stash.save

why: The code incorrectly passed "--quiet" as the message parameter,
creating stashes with literal "--quiet" message instead of suppressing output.
what:
- Change stash.save(message="--quiet") to stash.save(quiet=True)
- Remove unused git_stash_save_options variable
- Remove outdated Git < 1.7.6 comment

Author: tony

src/libvcs/sync/git.py

@@ -480,10 +480,8 @@ def update_repo(
             # If not in clean state, stash changes in order to be able
             # to be able to perform git pull --rebase
             if need_stash:
-                # If Git < 1.7.6, uses --quiet --all
-                git_stash_save_options = "--quiet"
                 try:
-                    process = self.cmd.stash.save(message=git_stash_save_options)
+                    process = self.cmd.stash.save(quiet=True)
                 except exc.CommandError:
                     self.log.exception("Failed to stash changes")