grokmirror: absolute paths and objstores..

[Eric Wong <e@80x24.org>]

One thing I noticed with grokmirror is that it uses absolute
paths everywhere.  That makes it non-trivial to relocate the repos
to a different FS location (e.g.: clone on my workstation while
on a fast network, copy to USB stick, copy to laptop, and work
offline)


I also wonder if using only URLs (and not FS paths) remote.*.url
in both objstores and non-objstores would be simpler and more
robust.  In that case, `git fetch --all' can be called from the
objstores to update all remotes.

The non-objstores would only store refs in nearly all cases.

That would completely avoid local FS paths from being in
$GIT_DIR/config, only objects/info/alternates

A lazy implementor can just call `git fetch' on the non-objstore
and let alternates + repack deal with redundancy in case there's
a small window where commits appear after the objstore fetch
but before the non-objstore fetch.

An implementor can of course avoid `git fetch' on non-objstores
by manually updating refs in the non-objstore, though.