Copy Rails credential keys into new worktrees during Superconductor setup

Worktrees forked from the main checkout were missing master.key and
environment credential keys, so the app couldn't decrypt credentials.
The setup step now copies them from the main repo into each worktree.

Author: Brian Casel

.superconductor/config.json

@@ -1,5 +1,6 @@
 {
   "setup": [
+    "SRC=\"$(cd \"$(dirname \"$(git rev-parse --git-common-dir)\")\" && pwd)\"; if [ \"$SRC\" != \"$PWD\" ]; then mkdir -p config/credentials; for f in master.key credentials/development.key credentials/production.key; do [ -f \"$SRC/config/$f\" ] && cp -f \"$SRC/config/$f\" \"config/$f\"; done; fi; true",
     "bin/setup",
     "npm install"
   ],