Add support for running self-tests when 9pm is a submodule

When working on 9PM while it is used as a GIT submodule in another
project, .git is not a directory.  Handle this case by looking up
the actual .git directory:

    $ ls -la .git
    -rw-rw-r-- 1 bob bob 31 dec  1  2023 .git
    $ cat .git
    gitdir: ../../.git/modules/9pm

Signed-off-by: Joachim Wiberg <[email protected]>

Author: troglobit

9pm.py

@@ -771,10 +771,20 @@ def setup_env(args):
         os.environ["NINEPM_CONFIG"] = args.config
 
 def run_git_cmd(path, command):
-    gitdir = os.path.join(path, '.git')
+    git_path = os.path.join(path, '.git')
 
-    if not os.path.isdir(gitdir):
-        vcprint(pcolor.orange, f"warning, no .git dir in path ({path})")
+    if os.path.isfile(git_path):
+        with open(git_path, 'r') as f:
+            line = f.read().strip()
+            if line.startswith('gitdir: '):
+                gitdir = os.path.join(path, line[8:])
+            else:
+                vcprint(pcolor.orange, f"warning, invalid .git file format ({path})")
+                return ""
+    elif os.path.isdir(git_path):
+        gitdir = git_path
+    else:
+        vcprint(pcolor.orange, f"warning, no .git dir or file in path ({path})")
         return ""
 
     try: