gpg-interface: simplify ssh fingerprint parsing

In "gpg-interface.c", the 'parse_ssh_output()' function takes a 'struct signature_check *sigc' argument and populates many members of this 'sigc' using information parsed from 'sigc->output' which contains the ouput of an `ssh-keygen -Y ...` command that was used to verify an SSH signature. When it populates 'sigc->fingerprint' though, it uses `xstrdup(strstr(line, "key ") + 4)` while `strstr(line, "key ")` has already been computed a few lines above and is already available in the `key` variable. Let's simplify this. Signed-off-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>