RF+BF: return full .dot_git path if .git is not subpath

In my previous refactoring, while reading the old get_git_dir implementation
that path would always be relative (since ".git" was returned etc).  But it
did resolve symlinks and read links from .git file.

This change does not necessarily makes it match 100%  old behavior, since
we would still return relative it if it points e.g. somewhere under .git somehow,
but as for the target need of discovering .git location I think it should
be functionally equivalent (I would have even stripped maybe_relative completely).

datalad/support/gitrepo.py

@@ -1182,15 +1182,15 @@ def is_valid_repo(cls, path):
         )
 
     @staticmethod
-    def _get_dot_git(pathobj, *, ok_missing=False, relative=False):
+    def _get_dot_git(pathobj, *, ok_missing=False, maybe_relative=False):
         """Given a pathobj to a repository return path to .git/ directory
 
         Parameters
         ----------
-        pathjob: Path
+        pathobj: Path
         ok_missing: bool, optional
           Allow for .git to be missing (useful while sensing before repo is initialized)
-        relative: bool, optional
+        maybe_relative: bool, optional
           Return path relative to pathobj
 
         Raises
@@ -1201,7 +1201,7 @@ def _get_dot_git(pathobj, *, ok_missing=False, relative=False):
         Returns
         -------
         Path
-          Absolute (unless relative=True) path to resolved .git/ directory
+          Absolute (unless maybe_relative=True) path to resolved .git/ directory
         """
         dot_git = pathobj / '.git'
         if dot_git.is_file():
@@ -1216,8 +1216,12 @@ def _get_dot_git(pathobj, *, ok_missing=False, relative=False):
         elif not (ok_missing or dot_git.exists()):
             raise RuntimeError("Missing .git in %s." % pathobj)
         # Primarily a compat kludge for get_git_dir, remove when it is deprecated
-        if relative:
-            dot_git = dot_git.relative_to(pathobj)
+        if maybe_relative:
+            try:
+                dot_git = dot_git.relative_to(pathobj)
+            except ValueError:
+                # is not a subpath, return as is
+                lgr.debug("Path %r is not subpath of %r", dot_git, pathobj)
         return dot_git
 
     @staticmethod
@@ -1246,7 +1250,7 @@ def get_git_dir(repo):
         """
         if isinstance(repo, GitRepo):
             return str(repo.dot_git)
-        return str(GitRepo._get_dot_git(Path(repo), ok_missing=False, relative=True))
+        return str(GitRepo._get_dot_git(Path(repo), ok_missing=False, maybe_relative=True))
 
     @property
     def config(self):