am: don't infloop for an empty input file

git-am.sh's check_patch_format function would attempt to preview
the patch to guess its format, but would go into an infinite loop
when the patch file happened to be empty.  The solution: exit the
loop when "read" fails, not when the line var, "$l1" becomes empty.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -201,7 +201,7 @@ check_patch_format () {
 		l1=
 		while test -z "$l1"
 		do
-			read l1
+			read l1 || break
 		done
 		read l2
 		read l3

t/t4150-am.sh

@@ -495,4 +495,14 @@ test_expect_success 'am -q is quiet' '
 	! test -s output.out
 '
 
+test_expect_success 'am empty-file does not infloop' '
+	rm -fr .git/rebase-apply &&
+	git reset --hard &&
+	touch empty-file &&
+	test_tick &&
+	{ git am empty-file > actual 2>&1 && false || :; } &&
+	echo Patch format detection failed. >expected &&
+	test_cmp expected actual
+'
+
 test_done