format-patch: dedup Cc list

Signed-off-by: John Keeping <john@keeping.me.uk>
rfc2822: extract header handling from mailinfo
Extract RFC2822 from builtin/mailinfo.c

Needs checking and possibly splitting into smaller commits.
format-patch: extract Cc recipients from in-reply-to message

By introducing a message store helper, we can look up a message's
content from its message ID, meaning that we can infer the "To" and "Cc"
headers from the original message instead of needing to specify them on
the command line.

Currently the body of the original message is ignored.

Signed-off-by: John Keeping <john@keeping.me.uk>
msgstore-gmane: new Gmane message store helper

This is a message store implementation that uses "mid.gmane.org" to find
a message in the mailing lists indexed by Gmane.

Signed-off-by: John Keeping <john@keeping.me.uk>