Protect against lost changes when setup_environment fails

Commit c3d972d fixed an issue where we would not restore changes in the
event a plugin failed to load. Unfortunately, it introduced a regression
where if `setup_environment` were to raise an error for any reason we
would always call `cleanup_environment`.

This is not necessarily always safe. For example, in the case of
pre-commit, `setup_environment` might fail before it is able to create a
stash of the working tree, so calling `cleanup_environment` could result
in lost changes.

Change the ordering so that we (hopefully) get the best of both worlds:

  * Call `load_hooks` first outside of the `begin`/`ensure` block so
    that if it fails we haven't touched the repository yet, so the repo
    won't be in a bad state if a plugin can't be loaded or a signature
    is invalid

  * Call `setup_environment` outside of the `begin`/`ensure` block so
    that in the worst case we don't accidentally clobber anything with
    `cleanup_environment`

Author: sds

lib/overcommit/hook_runner.rb

@@ -22,9 +22,20 @@ def run
       # these calls to allow some sort of "are you sure?" double-interrupt
       # functionality, but until that's deemed necessary let's keep it simple.
       InterruptHandler.isolate_from_interrupts do
+        # Load hooks before setting up the environment so that the repository
+        # has not been touched yet. This way any load errors at this point don't
+        # result in Overcommit leaving the repository in a bad state.
+        load_hooks
+
+        # Setup the environment without automatically calling
+        # `cleanup_environment` on an error. This is because it's possible that
+        # the `setup_environment` code did not fully complete, so there's no
+        # guarantee that `cleanup_environment` will be able to accomplish
+        # anything of value. The safest thing to do is therefore nothing in the
+        # unlikely case of failure.
+        @context.setup_environment
+
         begin
-          @context.setup_environment
-          load_hooks
           run_hooks
         ensure
           @context.cleanup_environment