bootstrap: check for git tree with .git/. in case of soft links.

* gl/build-aux/bootstrap.in (func_require_git): Use
`test -d .git/.` to defeat soft-links versus non-GNU test.
* bootstrap: Regenerate.
* THANKS: Remove duplicate Bruce Korb entry.

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