git-gui: ensure own version of git-gui--askpass is used

git-gui: ensure own version of git-gui--askpass is used

When finding a location for the askpass helper, git will be asked
for its exec path, but if that git is not the same that called
git-gui then we might mistakenly point to its helper instead.

Assume that git-gui and the helper are colocated to derive its
path instead.

This is specially useful in macOS where a broken version of that
helper is provided by the system git.

[j6t: move directory to variable to help in-flight topics]

Suggested-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>