am: support --show-current-patch=raw as a synonym for--show-current-patch

am: support --show-current-patch=raw as a synonym for--show-current-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 "git am --show-current-patch"
cannot be passed to "git apply" if it is encoded as quoted-printable or
base64.  To simplify worktree operations and to avoid that users poke into
.git, it would be better if "git am" also provided a mode that copies
.git/rebase-merge/patch to stdout.

One possibility could be to have completely separate options, introducing
for example --show-current-message (for .git/rebase-apply/NNNN)
and --show-current-diff (for .git/rebase-apply/patch), while possibly
deprecating --show-current-patch.

That would even remove the need for the first two patches in the series.
However, the long common prefix would have prevented using an abbreviated
option such as "--show".  Therefore, I chose instead to add a string
argument to --show-current-patch.  The new argument is optional, so that
"git am --show-current-patch"'s behavior remains backwards-compatible.

The next choice to make is how to handle multiple --show-current-patch
options.  Right now, something like "git am --abort --show-current-patch"
is rejected, and the previous suggestion would likewise have naturally
rejected a command line like

	git am --show-current-message --show-current-diff

Therefore, I decided to also reject for example

	git am --show-current-patch=diff --show-current-patch=raw

In other words the whole of --show-current-patch=xxx (including the
optional argument) is treated as the command mode.  I found this to be
more consistent and intuitive, even though it differs from the usual
"last one wins" semantics of the git command line.

Add the code to parse submodes based on the above design, where for now
"raw" is the only valid submode.  "raw" prints the full e-mail message
just like "git am --show-current-patch".

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>