checkpatch: allow not using -f with files that are in git

If a file exists in git and checkpatch is used without the -f flag for
scanning a file, then checkpatch will scan the file assuming it's a patch
and emit:

ERROR: Does not appear to be a unified-diff format patch

Change the behavior to assume the -f flag if the file exists in git.

[joe@perches.com: fix git "fatal" warning if file argument outside kernel tree]
  Link: https://lkml.kernel.org/r/b6afa04112d450c2fc120a308d706acd60cee294.camel@perches.comSigned-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Julia Lawall <julia.lawall@inria.fr>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lkml.kernel.org/r/45b81a48e1568bd0126a96f5046eb7aaae9b83c9.camel@perches.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -974,6 +974,16 @@ sub seed_camelcase_includes {
 	}
 }
 
+sub git_is_single_file {
+	my ($filename) = @_;
+
+	return 0 if ((which("git") eq "") || !(-e "$gitroot"));
+
+	my $output = `${git_command} ls-files -- $filename 2>/dev/null`;
+	my $count = $output =~ tr/\n//;
+	return $count eq 1 && $output =~ m{^${filename}$};
+}
+
 sub git_commit_info {
 	my ($commit, $id, $desc) = @_;
 
@@ -1047,6 +1057,9 @@ my $vname;
 $allow_c99_comments = !defined $ignore_type{"C99_COMMENT_TOLERANCE"};
 for my $filename (@ARGV) {
 	my $FILE;
+	my $is_git_file = git_is_single_file($filename);
+	my $oldfile = $file;
+	$file = 1 if ($is_git_file);
 	if ($git) {
 		open($FILE, '-|', "git format-patch -M --stdout -1 $filename") ||
 			die "$P: $filename: git format-patch failed - $!\n";
@@ -1091,6 +1104,7 @@ for my $filename (@ARGV) {
 	@modifierListFile = ();
 	@typeListFile = ();
 	build_types();
+	$file = $oldfile if ($is_git_file);
 }
 
 if (!$quiet) {