Fixes: lix-project/lix#157
Fixes: lix-project/lix#221
Previously the entire escaped store URI was included. This would cause
build failures if a very long or deeply nested path was being used in
the store.
Now, we use the first 48 characters of the URL (escaped), then 16 bytes
of hash of the entire URL. This should never collide and limits the
length of the file name to a bit over 64, which is fine.
Change-Id: Ic1ba690a94e83749567c2c29460b8d1bcf2ac413