format-patch: improve head calculation for cover-letter

If we do it after the revision traversal we can be sure that this is
indeed a commit that will be processed (i.e. not a merge) and it's the
top most one (thus removing the NEEDSWORK comment, at least we show the
same as 'git diff --stat' output that appears in the cover-letter).

While we are at it, since we know there's nothing to generate, exit
sooner in all cases, like --cover-letter currently does.

Also, if there's nothing to generate and cover-letter is specified, a
different code-path might be triggered that is not currently covered in
the test-case, so add a test for it.

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

builtin/log.c

@@ -1311,24 +1311,6 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	rev.show_root_diff = 1;
 
 	if (cover_letter) {
-		/*
-		 * NEEDSWORK:randomly pick one positive commit to show
-		 * diffstat; this is often the tip and the command
-		 * happens to do the right thing in most cases, but a
-		 * complex command like "--cover-letter a b c ^bottom"
-		 * picks "c" and shows diffstat between bottom..c
-		 * which may not match what the series represents at
-		 * all and totally broken.
-		 */
-		int i;
-		for (i = 0; i < rev.pending.nr; i++) {
-			struct object *o = rev.pending.objects[i].item;
-			if (!(o->flags & UNINTERESTING))
-				head = (struct commit *)o;
-		}
-		/* There is nothing to show; it is not an error, though. */
-		if (!head)
-			return 0;
 		if (!branch_name)
 			branch_name = find_branch_name(&rev);
 	}
@@ -1364,6 +1346,10 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		list = xrealloc(list, nr * sizeof(list[0]));
 		list[nr - 1] = commit;
 	}
+	if (nr == 0)
+		/* nothing to do */
+		return 0;
+	head = list[0];
 	total = nr;
 	if (!keep_subject && auto_number && total > 1)
 		numbered = 1;

t/t4014-format-patch.sh

@@ -1284,4 +1284,9 @@ test_expect_success 'cover letter using branch description (6)' '
 	grep hello actual >/dev/null
 '
 
+test_expect_success 'cover letter with nothing' '
+	git format-patch --stdout --cover-letter >actual &&
+	test_line_count = 0 actual
+'
+
 test_done