check-commit: print limit when user has a too-long summary

contrib/check-commit

@@ -27,7 +27,7 b' errors = ['
     (r"^# .*\n[A-Z][a-z]\S+", "don't capitalize summary lines"),
     (r"^# .*\n[^\n]*: *[A-Z][a-z]\S+", "don't capitalize summary lines"),
     (r"^# .*\n.*\.\s+$", "don't add trailing period on summary line"),
-    (r"^# .*\n.{78,}", "summary line too long"),
+    (r"^# .*\n.{78,}", "summary line too long (limit is 78)"),
     (r"^\+\n \n", "adds double empty line"),
     (r"\+\s+def [a-z]+_[a-z]", "adds a function with foo_bar naming"),
 ]