checkpatch: add an invalid patch separator test

Some versions of tools that apply patches incorrectly allow lines that
start with 3 dashes and have additional content on the same line.

Checkpatch will now emit an ERROR on these lines and optionally convert
those lines from dashes to equals with --fix.

Link: https://lkml.kernel.org/r/6ec1ed08328340db42655287afd5fa4067316b11.camel@perches.com
Signed-off-by: Joe Perches <joe@perches.com>
Suggested-by: Ian Rogers <irogers@google.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: Namhyung kim <namhyung@kernel.org>
Cc: Stehen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/dev-tools/checkpatch.rst

@@ -601,6 +601,11 @@ Commit message
 
     See: https://www.kernel.org/doc/html/latest/process/submitting-patches.html#describe-your-changes
 
+  **BAD_COMMIT_SEPARATOR**
+    The commit separator is a single line with 3 dashes.
+    The regex match is '^---$'
+    Lines that start with 3 dashes and have more content on the same line
+    may confuse tools that apply patches.
 
 Comparison style
 ----------------

scripts/checkpatch.pl

@@ -3031,6 +3031,16 @@ sub process {
 			}
 		}
 
+# Check for invalid patch separator
+		if ($in_commit_log &&
+		    $line =~ /^---.+/) {
+			if (ERROR("BAD_COMMIT_SEPARATOR",
+				  "Invalid commit separator - some tools may have problems applying this\n" . $herecurr) &&
+			    $fix) {
+				$fixed[$fixlinenr] =~ s/-/=/g;
+			}
+		}
+
 # Check for patch separator
 		if ($line =~ /^---$/) {
 			$has_patch_separator = 1;