precommit: terraform fmt makes changes on disk

 ### Issues Addressed
Most of our pre-commit checks just make the changes that are rote,
obvious changes. Not `terraform fmt`! And there's no reason for it, it's
just someone decided to do it that way and the maintainers won't switch
it back. Comment thread:
gruntwork-io#48

 ### Summary of Changes
I patched precommit to use the expected behavior. I included a shell
file to add the patch so it's easy to stay up-to-date with upstream.

 ### Test Plan
- Tested that precommit now automatically updates terraform files when
  using this branch for precommit.

hooks/terraform-fmt.sh

@@ -12,7 +12,7 @@ export PATH=$PATH:/usr/local/bin
 FMT_ERROR=0
 
 for file in "$@"; do
-  terraform fmt -diff -check "$file" || FMT_ERROR=$?
+  terraform fmt "$file" || FMT_ERROR=$?
 done
 
 # reset path to the original value

memfault-patch.sh

@@ -0,0 +1,3 @@
+#!/bin/bash
+
+sed -i -e 's/terraform fmt -diff -check/terraform fmt/' hooks/terraform-fmt.sh