build: mention where to find the invalid commit message, when validation fails (#32420)

Whenever someone tries to commit (by running `git commit` directly or
indirectly), a `commit-msg` git hook is run to validate the commit
message. If the validation fails, an error message is printed and the
commit is aborted.

Occasionally, people may have written a non-trivial commit message which
could turn out to be invalid (due a small typo for example). In that
case, it is frustrating to "lose" the whole message and have to write it
all over again (from memory). This is frustrating and has happened to me
enough times to finally seek a solution.

Fortunately, it turns out that git stores the last commit message in
`.git/COMMIT_EDITMSG`, so it is easy to get it back (as long as you know
where to look for it). This commit mentions this info in the validation
error to help people that might not know about it.

(This issue is probably mostly relevant for people using git from the
command-line and not through a UI, but it won't hurt in either case.)

PR Close #32420

scripts/git/commit-msg.js

@@ -24,7 +24,10 @@ if (msgFile) {
     isValid = checkMsg(firstLine);
 
     if (!isValid) {
-        console.error('\nCheck CONTRIBUTING.md at the root of the repo for more information.');
+        console.error(
+            '\nCheck CONTRIBUTING.md at the root of the repo for more information.' +
+            '\n' +
+            '\n(In case you need the invalid commit message, it should be stored in \'.git/COMMIT_EDITMSG\'.)');
     }
 }