Fix up path-cleanup in git_path() properly

Fix up path-cleanup in git_path() properly

GIT_DIR=.  ends up being what some of the pack senders use, and we
sometimes messed up when cleaning up the path, ie a ".//HEAD" was
cleaned up into "/HEAD", not "HEAD" like it should be.

We should do some other cleanup, and probably also verify that symlinks
don't point to outside the git area.