Could not find am-ready mbox for current buffer

[Leo <git@relevant-information.com>]

Hello,

This is really just for future reference, in case someone else stumbles
upon this.  I've resolved it for myself and am just documenting what the
issue was.


I found that for a specific email with a patch piem would just refuse to
apply it with the message "Could not find am-ready mbox for current
buffer", even if just piping the email through `git am` would work.
Turns out the issue was with piem-patch-subject-re.  The patch was sent
with a different subject prefix (the --subject-prefix option in
git-send-email) than the default, which didn't match
piem-patch-subject-re.

For example: 

> Subject: [RFC] add some code

So if you, like me, just can't figure out why piem is angry with just
this specific patch, consider adjusting that variable.

Cheers,

Leo