am: allow individual e-mail files as input

We traditionally allowed a mbox file or a directory name of a maildir (but
never an individual file inside a maildir) to be given to "git am".  Even
though an individual file in a maildir (or more generally, a piece of
RFC2822 e-mail) is not a mbox file, it contains enough information to
create a commit out of it, so there is no reason to reject one.  Running
mailsplit on such a file feels stupid, but it does not hurt.

This builds on top of a5a6755 (git-am foreign patch support: introduce
patch_format, 2009-05-27) that introduced mailbox format detection.  The
codepath to deal with a mbox requires it to begin with "From " line and
also allows it to begin with "From: ", but a random piece of e-mail can
and often do begin with any valid RFC2822 header lines.

Instead of checking the first line, we extract all the lines up to the
first empty line, and make sure they look like e-mail headers.

A test is added to t4150 to demonstrate this feature.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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