checkpatch: remove false warning for commit reference

Checkpatch warns of an incorrect commit reference style for any
hexadecimal number of 12 digits and more.

Numbers of 12 digits are not necessarily commit ids.

For an example provoking the problem see
  https://patchwork.kernel.org/patch/9170897/

Checkpatch should only warn if the number refers to an existing commit.

Link: http://lkml.kernel.org/r/20170607184008.5869-1-xypron.glpk@gmx.de
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -867,6 +867,7 @@ sub git_commit_info {
 #		    echo "commit $(cut -c 1-12,41-)"
 #		done
 	} elsif ($lines[0] =~ /^fatal: ambiguous argument '$commit': unknown revision or path not in the working tree\./) {
+		$id = undef;
 	} else {
 		$id = substr($lines[0], 0, 12);
 		$desc = substr($lines[0], 41);
@@ -2606,7 +2607,8 @@ sub process {
 			($id, $description) = git_commit_info($orig_commit,
 							      $id, $orig_desc);
 
-			if ($short || $long || $space || $case || ($orig_desc ne $description) || !$hasparens) {
+			if (defined($id) &&
+			   ($short || $long || $space || $case || ($orig_desc ne $description) || !$hasparens)) {
 				ERROR("GIT_COMMIT_ID",
 				      "Please use git commit description style 'commit <12+ chars of sha1> (\"<title line>\")' - ie: '${init_char}ommit $id (\"$description\")'\n" . $herecurr);
 			}