checkpatch, SubmittingPatches: suggest line wrapping commit messages at 75 columns

Commit messages lines are sometimes overly long.

Suggest line wrapping at 75 columns so the default git commit log
indentation of 4 plus the commit message text still fits on an 80 column
screen.

Add a checkpatch test for long commit messages lines too.
Signed-off-by: Joe Perches <joe@perches.com>
Cc: David Miller <davem@davemloft.net>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ian Morris <ipm@chirality.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/SubmittingPatches

@@ -614,8 +614,8 @@ The canonical patch message body contains the following:
 
   - An empty line.
 
-  - The body of the explanation, which will be copied to the
-    permanent changelog to describe this patch.
+  - The body of the explanation, line wrapped at 75 columns, which will
+    be copied to the permanent changelog to describe this patch.
 
   - The "Signed-off-by:" lines, described above, which will
     also go in the changelog.

scripts/checkpatch.pl

@@ -1898,6 +1898,7 @@ sub process {
 
 	my $in_header_lines = $file ? 0 : 1;
 	my $in_commit_log = 0;		#Scanning lines before patch
+	my $commit_log_long_line = 0;
 	my $reported_maintainer_file = 0;
 	my $non_utf8_charset = 0;
 
@@ -2233,6 +2234,14 @@ sub process {
 			      "Remove Gerrit Change-Id's before submitting upstream.\n" . $herecurr);
 		}
 
+# Check for line lengths > 75 in commit log, warn once
+		if ($in_commit_log && !$commit_log_long_line &&
+		    length($line) > 75) {
+			WARN("COMMIT_LOG_LONG_LINE",
+			     "Possible unwrapped commit description (prefer a maximum 75 chars per line)\n" . $herecurr);
+			$commit_log_long_line = 1;
+		}
+
 # Check for git id commit length and improperly formed commit descriptions
 		if ($in_commit_log && $line =~ /\b(c)ommit\s+([0-9a-f]{5,})/i) {
 			my $init_char = $1;