Merge branch 'sb/dashless'

* sb/dashless:
  Make usage strings dash-less
  t/: Use "test_must_fail git" instead of "! git"
  t/test-lib.sh: exit with small negagive int is ok with test_must_fail

Conflicts:
	builtin-blame.c
	builtin-mailinfo.c
	builtin-mailsplit.c
	builtin-shortlog.c
	git-am.sh
	t/t4150-am.sh
	t/t4200-rerere.sh

t/t7600-merge.sh

@@ -126,7 +126,7 @@ verify_merge() {
 		echo "[OOPS] unmerged files"
 		false
 	fi &&
-	if ! git diff --exit-code
+	if test_must_fail git diff --exit-code
 	then
 		echo "[OOPS] working tree != index"
 		false