git-gui: Add a dialog that shows the OpenSSH public key.

git-gui: Add a dialog that shows the OpenSSH public key.

Generating a new SSH key or finding an existing one may
be a difficult task for non-technical users, especially
on Windows.

This commit adds a new dialog that shows the public key,
or allows the user to generate a new one if none were found.
Since this is a convenience/informational feature for new
users, and the dialog is mostly read-only, it is located
in the Help menu.

The command line used to invoke ssh-keygen is designed to
force it to use SSH_ASKPASS if available, or accept empty
passphrases, but _never_ wait for user response on the tty.

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>