Generalization of the `.git/MERGE_MSG' path

In general the `MERGE_MSG' file doesn't need to be located under the
local `.git' directory. In these cases the script `ciq-cherry-pick.py'
will fail.

An example would be a git repository created with `git-worktree'. Assume
the current directory contains a regular git repository `some-project'
with a branch `branch-x' and some commit with a hash
‹some-commit›. Without this change the last command would fail while it
shouldn't

,----
| cd some-project
| git worktree add ../some-project-branch-x  branch-x
| cd ../some-project-branch-x
| python3 ‹kernel-src-tree-tools-dir›/ciq-cherry-pick.py --sha ‹some-commit›
`----

In case of the `some-project-branch-x' repoitory the `MERGE_MSG' file is
under `some-project/.git/worktrees/some-project-branch-x/MERGE_MSG'.

Author: pvts-mat

ciq-cherry-pick.py

@@ -1,11 +1,12 @@
 import argparse
 import os
 import subprocess
+import git
 from ciq_helpers import CIQ_cherry_pick_commit_standardization
 from ciq_helpers import CIQ_original_commit_author_to_tag_string
 # from ciq_helpers import *
 
-MERGE_MSG = '.git/MERGE_MSG'
+MERGE_MSG = git.Repo(os.getcwd()).git_dir + '/MERGE_MSG'
 
 if __name__ == '__main__':
     print("CIQ custom cherry picker")