git-gui: Add a simple implementation of SSH_ASKPASS.

git-gui: Add a simple implementation of SSH_ASKPASS.

OpenSSH allows specifying an external program to use
for direct user interaction. While most Linux systems
already have such programs, some environments, for
instance, msysgit, lack it. This patch adds a simple
fallback Tcl implementation of the tool.

In msysgit it is also necessary to set a fake value of
the DISPLAY variable, because otherwise ssh won't even
try to use SSH_ASKPASS handlers.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Acked-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>