Honor url.insteadOf when setting up SSH control master connection

Repo can now properly handle url.insteadOf sections in the
user's ~/.gitconfig file.  This means that a user can now enjoy
the master-ssh functionality even if he/she uses insteadOf's in
~/.gitconfig to rewrite git:// URLs to ssh:// style URLs.

Change-Id: Ic0f04a9c57206a7b89eb0f10bf188c4c483debe3
Signed-off-by: Shawn O. Pearce <sop@google.com>