Make git-send-email detect mbox-style patches more readily

Make git-send-email detect mbox-style patches more readily

Earlier we insisted that mbox file to begin with "From ".  That
is fine as long as you feed format-patch output, but if you
handcraft the input file, this is unnecessary burden.  We should
detect lines that look like e-mail headers and say that is also
a mbox file.

The other input file format is traditional "send lots of email",
whose first line would never look like e-mail headers, so this
is a safe change.

The original patch was done by Matthew Wilcox, which checked
explicitly for headers the script pays attention to.

Signed-off-by: Junio C Hamano <junkio@cox.net>