ident: trim trailing newline from /etc/mailname

ident: trim trailing newline from /etc/mailname

We use fgets to read the /etc/mailname file, which means we
will typically end up with an extra newline in our
git_default_email. Most of the time this doesn't matter, as
fmt_ident will skip it as cruft, but there is one code path
that accesses it directly (in http-push.c:lock_remote).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>