33391: vcs_info git: fix applied patch detection on git am

git-am also uses .git/rebase-apply for patch list but
the file original-commit does not exist (as no commit exist).
This patch handles both git rebase and git am. Also:
 - get the first line (rather than the first char) when the message
   contains only one line;
 - remove unused function (ironically that should have been used here).