give a hint/solution for "It looks like git-am is in progress. Cannot rebase."

Hi,

I ran git-am instead of git-apply on my repo. The git-am complained about the input data, I completely ignored it because I never use git-am (except on typos + tab-completions like today).

Later, after a few commits I wanted to rebase (interactively) my tree
to merge a few commits.
  $ git rebase -i HEAD~4
  It looks like git-am is in progress. Cannot rebase.

Ok, the message is nice, it tells me it can't do something, but I'm stupid enough not to know how to proceed. I see that this message is the same in HEAD (master and next).

It would be nice if git would output one or two hints. For example:
* run "git cancel-a-git-am-in-progress" to cancel a git-am in progress
* run "git commit-the-git-am-in-progress" to commit the git-am in progress

Of course, the commands need to be replaced with some valid git commands, but I don't know which those commands are :)

The message is generated in git-rebase.sh by this code:
  test -f "$GIT_DIR"/rebase-apply/applying &&
  die 'It looks like git-am is in progress. Cannot rebase.'

For now I think I'll `rm -rf .git/rebase-apply` (and hopefully not break anything).