combine-diff: treat --dirstat like --stat

Currently "--cc --dirstat" will show nothing for a merge.  Like
--shortstat and --summary in the previous two patches, it probably makes
sense to treat it like we do --stat, and show a stat against the
first-parent.

This case is less obviously correct than for --shortstat and --summary,
as those are basically variants of --stat themselves. It's possible we
could develop a multi-parent combined dirstat format, in which case we
might regret defining this first-parent behavior. But the same could be
said for --stat, and in the 12+ years of it showing first-parent stats,
nobody has complained.

So showing the first-parent dirstat is at least _useful_, and if we
later develop a clever multi-parent stat format, we'd probably have to
deal with --stat anyway.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

combine-diff.c

@@ -1327,6 +1327,7 @@ static const char *path_path(void *obj)
 #define STAT_FORMAT_MASK (DIFF_FORMAT_NUMSTAT \
 			  | DIFF_FORMAT_SHORTSTAT \
 			  | DIFF_FORMAT_SUMMARY \
+			  | DIFF_FORMAT_DIRSTAT \
 			  | DIFF_FORMAT_DIFFSTAT)
 
 /* find set of paths that every parent touches */

t/t4013-diff-various.sh

@@ -358,6 +358,7 @@ diff --line-prefix=abc master master^ side
 diff --dirstat master~1 master~2
 diff --dirstat initial rearrange
 diff --dirstat-by-file initial rearrange
+diff --dirstat --cc master~1 master
 # No-index --abbrev and --no-abbrev
 diff --raw initial
 :noellipses diff --raw initial

t/t4013/diff.diff_--dirstat_--cc_master~1_master

@@ -0,0 +1,3 @@
+$ git diff --dirstat --cc master~1 master
+  40.0% dir/
+$