patch: support "patch [FILE [PATCH]]" format

function                                             old     new   delta
xopen_stdin                                            -      15     +15
patch_main                                          2075    2041     -34

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

testsuite/patch.tests

@@ -129,7 +129,6 @@ abc
 " \
 
 # testing "test name" "command(s)" "expected result" "file input" "stdin"
-
 testing "patch -N ignores already applied hunk" \
 	'patch -N 2>&1; echo $?; cat input' \
 "\
@@ -153,6 +152,29 @@ def
  123
 " \
 
+# testing "test name" "command(s)" "expected result" "file input" "stdin"
+testing "patch FILE PATCH" \
+	'cat >a.patch; patch input a.patch 2>&1; echo $?; cat input; rm a.patch' \
+"\
+patching file input
+0
+abc
+def
+123
+" \
+"\
+abc
+123
+" \
+"\
+--- foo.old
++++ foo
+@@ -1,2 +1,3 @@
+ abc
++def
+ 123
+" \
+
 rm input.orig 2>/dev/null
 
 exit $FAILCOUNT