test-fb-hgext-treemanifest-server: attempt to fix '[[' not found error

Summary:
This is an attempt to fix the following error:

```
 --- test-fb-hgext-treemanifest-server.t
+++ test-fb-hgext-treemanifest-server.t.simplecachestore.err
@@ -167,8 +167,9 @@
   $ hg push --to mybook
   pushing to ssh://user@dummy/master
   searching for changes
-  remote: prepushrebase.myhook hook exited with status 1
+  remote: prepushrebase.myhook hook exited with status 2
   abort: push failed on remote
+  remote: $TESTTMP/myhook.sh: 2: $TESTTMP/myhook.sh: [[: not found
   [255]
```

which I suspect is caused by a non-bash default shell.

Reviewed By: xavierd

Differential Revision: D19887222

fbshipit-source-id: b6fe5d89e4c41ff49fca86da927c4e702ed1e7c1

eden/scm/tests/test-fb-hgext-treemanifest-server.t

@@ -145,6 +145,7 @@ Test pushing only flat fails if forcetreereceive is on
 
 Test pushing flat and tree
   $ cat >> $TESTTMP/myhook.sh <<EOF
+  > #!/bin/bash
   > set -e
   > [[ \$(hg log -r \$HG_NODE -T '{file_adds}') == 'subdir2/z' ]] && exit 1
   > exit 2