doc: doc-diff: specify date

Earlier we changed the manual page formatting machinery to use the
dates from the commit the documentation source was taken from,
instead of the date the manual page was produced.  When "doc-diff"
compares two commits from different dates, the different dates from
the two commits would result in unnecessary differences in the
output because of the change.

Compensate by setting a fixed date when "doc-diff" formats the pages
to be compared to work around this issue.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/doc-diff

@@ -153,6 +153,7 @@ render_tree () {
 		make -j$parallel -C "$tmp/worktree" \
 			$makemanflags \
 			GIT_VERSION=omitted \
+			GIT_DATE=1970-01-01 \
 			SOURCE_DATE_EPOCH=0 \
 			DESTDIR="$tmp/installed/$dname+" \
 			install-man &&