clang-format: improve error message when CI fails

Don't just tell the problem but hint how to fix it.

Change-Id: I9d079ee7d4ed61266e22a3fa21efe10366724645
Reviewed-on: https://gerrit.libreoffice.org/49471Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>

.git-hooks/pre-commit

@@ -204,6 +204,8 @@ sub check_style($)
             print("and the clang-format rules. Fixing these now automatically.\n");
             print("Running '$clang_format -i " . join(" ", @bad_names) . "' for you...\n");
             system("$clang_format -i " . join(" ", @bad_names));
+            # TODO this stages all local modifications, staging originally
+            # unstaged hunks.
             system("git add " . join(" ", @bad_names));
             print("Done.\n");
         }

solenv/clang-format/check-last-commit

@@ -60,7 +60,10 @@ sub check_style()
     if (scalar @bad_names)
     {
         print("\nERROR: The above differences were found between the code to commit \n");
-        print("and the clang-format rules.\n");
+        print("and the clang-format rules. Tips:\n");
+        print("\n- You may run '/opt/lo/bin/clang-format -i <problematic file>' to fix up style automatically.\n");
+        print("- See solenv/clang-format/README on where to get the required version of clang-format binaries.\n");
+        print("- If you renamed a blacklisted file, update solenv/clang-format/blacklist accordingly to keep it blacklisted.\n");
         print("\nsolenv/clang-format/check-last-commit: KO\n");
         exit(1);
     }