make-version.pl: make it work with git worktrees

make-version.pl: make it work with git worktrees

When using git worktrees, the .git file is a regular file pointing to
the original git repository. Accept this case too. Also ignore the fact
that the original repo could be a git-svn repo, that is very unlikely.

Example workflow (requires git 2.5):

    # Assume work in progress in current tree. Goal is to quickly do a
    # fix in another branch and publish the changes without interfering
    # with the WIP and without marking all files out of date (which
    # slows down re-compilation).
    git worktree add /tmp/wireshark-2.2 master-2.2
    pushd /tmp/wireshark-2.2        # go to temporary tree
    git cherry-pick -x COMMIT       # backport the fix
    git review                      # submit for review
    popd                            # go back to original tree
    rm -rf /tmp/wireshark-2.2 && git worktree prune     # cleanup
    # now continue working where you left work

Change-Id: Iba3c723142d9cbd8b07e76647594b5699ecafc67
Reviewed-on: https://code.wireshark.org/review/17002
Reviewed-by: Jaap Keuter <jaap.keuter@xs4all.nl>
Petri-Dish: Jaap Keuter <jaap.keuter@xs4all.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>