am: teach mercurial patch parser how to read from stdin

git-mailsplit, which splits mbox patches, will read the patch from stdin
when the filename is "-" or there are no files listed on the
command-line.

To be consistent with this behavior, teach the mercurial patch parser to
read from stdin if the filename is "-" or no files are listed on the
command-line.

Based-on-patch-by: Chris Packham <judge.packham@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -327,6 +327,7 @@ split_patches () {
 		;;
 	hg)
 		this=0
+		test 0 -eq "$#" && set -- -
 		for hg in "$@"
 		do
 			this=$(( $this + 1 ))
@@ -353,7 +354,7 @@ split_patches () {
 					print "\n", $_ ;
 					$subject = 1;
 				}
-			' <"$hg" >"$dotest/$msgnum" || clean_abort
+			' -- "$hg" >"$dotest/$msgnum" || clean_abort
 		done
 		echo "$this" >"$dotest/last"
 		this=

t/t4150-am.sh

@@ -259,6 +259,16 @@ test_expect_success 'am applies hg patch' '
 	test_cmp_rev second^ HEAD^
 '
 
+test_expect_success 'am --patch-format=hg applies hg patch' '
+	rm -fr .git/rebase-apply &&
+	git checkout -f first &&
+	git am --patch-format=hg <patch1-hg.eml &&
+	test_path_is_missing .git/rebase-apply &&
+	git diff --exit-code second &&
+	test_cmp_rev second HEAD &&
+	test_cmp_rev second^ HEAD^
+'
+
 test_expect_success 'setup: new author and committer' '
 	GIT_AUTHOR_NAME="Another Thor" &&
 	GIT_AUTHOR_EMAIL="a.thor@example.com" &&