git_connect(): factor out SSH variant handling

git_connect(): factor out SSH variant handling

We handle plink and tortoiseplink as OpenSSH replacements, by passing
the correct command-line options when detecting that they are used.

To let users override that auto-detection (in case Git gets it wrong),
we need to introduce new code to that end.

In preparation for this code, let's factor out the SSH variant handling
into its own function, handle_ssh_variant().

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>