buildconfigs/src.git-clone: use git clone, avoid git remote

buildconfigs/src.git-clone: use git clone, avoid git remote

git 1.4.x (which is on even some moderately recent systems) doesn't
have the `git remote' command.  This is what caused the most recent
patchman build failure.

The machinery in buildconfigs/src.git-clone was rather more
complicated than needed, too.  In this patch I replace this with a
similar set of runes to those used for ioemu-remote (almost identical
except that the QEMU_TAG feature is not supported in this instance).

I have done a successful test build with this change.

Cc: Ian Campbell <Ian.Campbell@eu.citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>