am: support --show-current-patch=diff to retrieve .git/rebase-apply/patch

am: support --show-current-patch=diff to retrieve .git/rebase-apply/patch

When "git am --show-current-patch" was added in commit 984913a210 ("am:
add --show-current-patch", 2018-02-12), "git am" started recommending it
as a replacement for .git/rebase-merge/patch.  Unfortunately the suggestion
is somewhat misguided; for example, the output of "git am --show-current-patch"
cannot be passed to "git apply" if it is encoded as quoted-printable
or base64.  Add a new mode to "git am --show-current-patch" in order to
straighten the suggestion.

Reported-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>