send-email: work around double encoding of in-body From field.

send-email: work around double encoding of in-body From field.

git-send-email sends out the message taken from format-patch
output without quoting nor encoding.  When copying the From:
line to form in-body From: field, it should not copy it
verbatim, because the From: for the header is quoted according
to RFC 2047 when not ASCII.

The original came from Jürgen Rühle, but I moved the
string munging into a separate function so that later other
people can tweak it more easily.  Bugs introduced during the
translation are mine.

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