[notmuch] Notmuch-side support for git send-email --notmuch id:<MESSAGE-ID>

[Jed Brown]

These two patches provide support for features like the one in the
subject line.  Along with these two patches, I have one for git
format-patch (which I will submit upstream soon) that uses the output
from notmuch reply --format=headers-only to build a reply with proper
referencing, To, and Cc fields.

Since options for git send-email are passed to git format-patch,

  git send-email --notmuch id:<ID>

is a substitute for

  git send-email --in-reply-to <ID> --to <TO> --cc <CC> --add-header 'References: ...'