Fixed VCS detection in git worktrees

[peci1]

When running catkin_prepare_release in a Git directory which is a worktree, I get:

$ catkin_prepare_release --bump major
Prepare the source repository for a release.
Could not determine repository type of '.'

This is because the VCS detection expects .git to be a folder. However, in a worktree, this is not true.

$ file .git
.git: ASCII text

$ LANG=C ll .git
-rw-rw-r-- 1 peci1 peci1 71 Jul 16 15:03 .git

$ cat .git
gitdir: /media/data/subt/common/src/ros-utils/.git/worktrees/ros-utils

This PR fixes the detection. I'm not sure what unexpected consequences it can have, but I guess quite minor.