git: Show patch of the changes in commit message

The changes are only shown in the editor and do not land in the final
commit message.

For that setting the git-commit-last-msg function and the commit-msg
hook had to be updated.
The function is now a standalone function instead of anonymous and
uses every line until the first comment in COMMIT_EDITMSG discarding the
new information too.
The hook breaks now when checking line lengths when the changes start
since for some weird reason they are passed together with the rest of
the message instead of being deleted like the comments.

meta/git/hooks/commit-msg

@@ -42,7 +42,13 @@ done
 BKP_IFS="$IFS"
 IFS='
 '
+# The -v flag or commit.verbose setting add the changes of the commit to the
+# bottom of the commit message. But when the changes contain lines longer than
+# 72 characters this hook will fail when not breaking the loop before the end
+# start of the patch.
+verbose_start_line="$(git diff --staged -u | head -1)"
 for line in $body; do
+	[[ "$line" != "$verbose_start_line" ]] || break
 	[[ ${#line} -le 72 ]] || die "Body lines too long. (<= 72)\n"
 done
 IFS="$BKP_IFS"