gpg-interface: simplify ssh fingerprint parsing

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>