GIT-VERSION-GEN: fix overriding GIT_BUILT_FROM_COMMIT and GIT_DATE

GIT-VERSION-GEN: fix overriding GIT_BUILT_FROM_COMMIT and GIT_DATE

Same as with the preceding commit, neither GIT_BUILT_FROM_COMMIT nor
GIT_DATE can be overridden via the environment. Especially the latter is
of importance given that we set it in our own "Documentation/doc-diff"
script.

Make the values of both variables overridable. Luckily we don't pull in
these values via any included Makefiles, so the fix is trivial compared
to the fix for GIT_VERSON.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>