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>

Author: pks-t

GIT-VERSION-GEN

@@ -56,8 +56,16 @@ then
 	GIT_VERSION=$(expr "$VN" : v*'\(.*\)')
 fi
 
-GIT_BUILT_FROM_COMMIT=$(git -C "$SOURCE_DIR" rev-parse -q --verify HEAD 2>/dev/null)
-GIT_DATE=$(git -C "$SOURCE_DIR" show --quiet --format='%as' 2>/dev/null)
+if test -z "$GIT_BUILT_FROM_COMMIT"
+then
+	GIT_BUILT_FROM_COMMIT=$(git -C "$SOURCE_DIR" rev-parse -q --verify HEAD 2>/dev/null)
+fi
+
+if test -z "$GIT_DATE"
+then
+	GIT_DATE=$(git -C "$SOURCE_DIR" show --quiet --format='%as' 2>/dev/null)
+fi
+
 if test -z "$GIT_USER_AGENT"
 then
 	GIT_USER_AGENT="git/$GIT_VERSION"

shared.mak

@@ -121,6 +121,8 @@ endef
 # absolute path to the root source directory as well as input and output files
 # as arguments, in that order.
 define version_gen
+GIT_BUILT_FROM_COMMIT="$(GIT_BUILT_FROM_COMMIT)" \
+GIT_DATE="$(GIT_DATE)" \
 GIT_USER_AGENT="$(GIT_USER_AGENT)" \
 GIT_VERSION="$(GIT_VERSION_OVERRIDE)" \
 $(SHELL_PATH) "$(1)/GIT-VERSION-GEN" "$(1)" "$(2)" "$(3)"