Allow difffing of rails credential files

From:
https://stackoverflow.com/questions/49514426/how-can-i-git-diff-changes-to-rails-5-2-credentials/50466819#50466819

gitattributes_global

@@ -0,0 +1 @@
+*.yml.enc diff=enc

gitconfig

@@ -34,6 +34,7 @@
 [core]
   autocrlf = input
   excludesfile = ~/.gitignore_global
+  attributesfile = ~/.gitattributes_global
   filemode = false
 [gpg]
 	program = gpg
@@ -47,3 +48,6 @@
 [transfer]
   fsckobjects = false
   # https://groups.google.com/forum/#!topic/binary-transparency/f-BI4o8HZW0
+[diff "enc"]
+  textconv = bundle exec rails encrypted:show
+  cachetextconv = false