git-gui - use git-hook, honor core.hooksPath

git-gui - use git-hook, honor core.hooksPath

git-gui currently runs some hooks directly using its own code written
before 2010, long predating git v2.9 that added the core.hooksPath
configuration to override the assumed location at $GIT_DIR/hooks.  Thus,
git-gui looks for and runs hooks including prepare-commit-msg,
commit-msg, pre-commit, post-commit, and post-checkout from
$GIT_DIR/hooks, regardless of configuration. Commands (e.g., git-merge)
that git-gui invokes directly do honor core.hooksPath, meaning the
overall behaviour is inconsistent.

Furthermore, since v2.36 git exposes its hook execution machinery via
`git-hook run`, eliminating the need for others to maintain code
duplicating that functionality.  Using git-hook will both fix git-gui's
current issues on hook configuration and (presumably) reduce the
maintenance burden going forward. So, teach git-gui to use git-hook.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>