git-gui: use git_read in githook_read

git-gui: use git_read in githook_read

0730a5a3a5e6 ("git-gui - use git-hook, honor core.hooksPath", 2023-09-17)
rewrote githook_read to use `git hook` to run a hook script. The code
that was replaced discovered the hook script file manually and invoked
it using function _open_stdout_stderr. After the rewrite, this function
is still invoked, but it calls into `git` instead of the hook scripts.

Notice though, that we have function git_read that invokes git and
prepares a pipe for the caller to read from. Replace the implementation
of githook_read to be just a wrapper around git_read. This unifies the
way in which the git executable is invoked. git_read ultimately also
calls into _open_stdout_stderr, but it modifies the path to the git
executable before doing so.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Taylor Blau <me@ttaylorr.com>