bootstrap: use `-d .git` to check whether we are in a git tree.

* gl/build-aux/bootstrap.in (func_require_git): .git is not a
regular file, use -d to check its existence.
* bootstrap: Regenerate.
* THANKS: Add Bruce Korb.
Reported by Bruce Korb

Copyright-paperwork-exempt: Yes
Signed-off-by: Gary V. Vaughan <gary@gnu.org>