pre-commit hook: complain about conflict markers.

Several <<< or === or >>> characters at the beginning of a line
is very likely to be leftover conflict markers from a failed
automerge the user resolved incorrectly, so detect them.

As usual, this can be defeated with "git commit --no-verify" if
you really do want to have those files, just like changes that
introduce trailing whitespaces.

Signed-off-by: Junio C Hamano <junkio@cox.net>

templates/hooks--pre-commit

@@ -61,6 +61,9 @@ perl -e '
 	    if (/^\s* 	/) {
 		bad_line("indent SP followed by a TAB", $_);
 	    }
+	    if (/^(?:[<>=]){7}/) {
+		bad_line("unresolved merge conflict", $_);
+	    }
 	}
     }
     exit($found_bad);