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.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

GIT-VERSION-GEN

@@ -53,10 +53,18 @@ then
 else
 	VN="$DEF_VER"
 fi
-
 GIT_VERSION=$(expr "$VN" : v*'\(.*\)')
-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"