Client-side mirroring patches (v0)

Hey folks, this is the first stage of git mirroring - making the
client support it in the face of a completely ignorant server.  I
intended to make the clean-up better, but I've been sitting on these
for a couple of weeks so I thought it would be better to have them out
there for people to have a squiz at.

The next stage would be for the mirror list to be communicated to
clients over the network protocol and updated in the git config.

Also there is the matter of falling over to the next mirror should one
not be reachable, but then we're getting into C weaknesses really.
Should I plan to do exception recovery using 'longjmp' ?  Also the
process should be interruptible and provide a user menu.  Again this
seems like it would be very tedious and clumsy in C.  How do people
manage?  Anyway, enjoy... Sam