git-gui: make output of GIT-VERSION-GEN source'able

git-gui: make output of GIT-VERSION-GEN source'able

The output of GIT-VERSION-GEN can be sourced by our Makefile to make the
version available there. The output has a couple of spaces around the
equals sign, which is perfectly valid for parsing it in our Makefile.
But in subsequent steps we'll also want to source the file in a couple
of newly-introduced shell scripts, but having spaces around variable
assignments is invalid there.

Prepare for this step by dropping the spaces surrounding the equals
sign. Like this, we can easily use the same file both in our Makefile
and in shell scripts.

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