git-diff: turn on recursion by default

The tree recursion behavior of git-diff may appear
inconsistent to the user because it depends on the format of
the patch as well as whether one is diffing between trees or
against the index.

Since git-diff is a porcelain wrapper for low-level diff
commands, it makes sense for its behavior to be consistent
no matter what is being diffed.  This patch turns on
recursion in all cases.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-diff.c

@@ -233,6 +233,7 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 			die("diff_setup_done failed");
 	}
 	rev.diffopt.allow_external = 1;
+	rev.diffopt.recursive = 1;
 
 	/* Do we have --cached and not have a pending object, then
 	 * default to HEAD by hand.  Eek.

t/t9300-fast-import.sh

@@ -577,7 +577,7 @@ EXPECT_END
 test_expect_success \
     'L: verify internal tree sorting' \
 	'git-fast-import <input &&
-	 git diff --raw L^ L >output &&
+	 git diff-tree --abbrev --raw L^ L >output &&
 	 git diff expect output'
 
 ###