ident: do not drop username when reading from /etc/mailname

ident: do not drop username when reading from /etc/mailname

An earlier conversion from fgets() to strbuf_getline() in the
codepath to read from /etc/mailname to learn the default host-part
of the ident e-mail address forgot that strbuf_getline() stores the
line at the beginning of the buffer just like fgets().

The "username@" the caller has prepared in the strbuf, expecting the
function to append the host-part to it, was lost because of this.

Reported-by: Mihai Rusu <dizzy@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>