git-gui: prepare GIT-VERSION-GEN for out-of-tree builds

git-gui: prepare GIT-VERSION-GEN for out-of-tree builds

The GIT-VERSION-GEN unconditionally writes version information into the
source directory in the form of the "GIT-VERSION-FILE". We are about to
introduce the Meson build system though, which enforces out-of-tree
builds by default, and in that context we cannot continue to write
version information into the source tree.

Prepare the script for out-of-tree builds by treating the source
directory different from the output file.

Signed-off-by: Patrick Steinhardt <ps@pks.im>