format-patch: fix From header in cover letter

format-patch: fix From header in cover letter

"git format-patch" takes "--from=<user ident>" command line option
and uses the given ident for patch e-mails, but this is not applied
to the cover letter, the option is ignored and the committer ident
of the current user is used. This has been the case ever since
"--from" was introduced in a9080475 (teach format-patch to place
other authors into in-body "From", 2013-07-03).

Teach the make_cover_letter() function to honor the option, instead of
always using the current committer identity. Change variable name from
"committer" to "from" to better reflect the purpose of the variable.

Signed-off-by: Mirko Faina <mroik@delayed.space>
Signed-off-by: Junio C Hamano <gitster@pobox.com>