git-drover: Correctly resolve --parent_checkout

git rev-parse --git-dir returns a relative path, e.g. ".git". This
should be resolved relative to the cwd of the git command instead of the
current working dir. The cwd of the git command is set to _parent_repo
(=the absolute path after resolving --parent_checkout), so use that.

TEST=The following completes without errors.
$ cd /tmp
$ git init somegitrepo
$ cd somegitrepo
$ git drover --branch [branch] --cherry-pick [hash] -v --dry-run --parent_checkout /path/to/src
$ cd /path/to/src
$ git drover --branch [branch] --cherry-pick [hash] -v --dry-run

Review URL: https://codereview.chromium.org/1640233002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@298416 0039d316-1c4b-4281-b951-d872f2087c98

git_drover.py

@@ -227,7 +227,7 @@ class _Drover(object):
     This is so the new workdir can be a sparse checkout without affecting
     |self._parent_repo|.
     """
-    parent_git_dir = os.path.abspath(self._run_git_command(
+    parent_git_dir = os.path.join(self._parent_repo, self._run_git_command(
         ['rev-parse', '--git-dir']).strip())
     self._workdir = tempfile.mkdtemp(prefix='drover_%s_' % self._branch)
     logging.debug('Creating checkout in %s', self._workdir)