LocalStore: Allow the physical and logical store directories to differ

This is primarily to subsume the functionality of the
copy-from-other-stores substituter. For example, in the NixOS
installer, we can now do (assuming we're in the target chroot, and the
Nix store of the installation CD is bind-mounted on /tmp/nix):

  $ nix-build ... --option substituters 'local?state=/tmp/nix/var&real=/tmp/nix/store'

However, unlike copy-from-other-stores, this also allows write access
to such a store. One application might be fetching substitutes for
/nix/store in a situation where the user doesn't have sufficient
privileges to create /nix, e.g.:

  $ NIX_REMOTE="local?state=/home/alice/nix/var&real=/home/alice/nix/store" nix-build ...

1 files changed, 0 insertions, 1 deletions

diff --git a/.gitignore b/.gitignore
index a175e8dfe291..178783d22d36 100644
--- a/.gitignore
+++ b/.gitignore
@@ -44,7 +44,6 @@ Makefile.config
 /scripts/nix-copy-closure
 /scripts/NixConfig.pm
 /scripts/NixManifest.pm
-/scripts/copy-from-other-stores.pl
 /scripts/download-from-binary-cache.pl
 /scripts/find-runtime-roots.pl
 /scripts/build-remote.pl