ignore .merlin files

Summary: prevent `.merlin` files from being managed by mercrial

Reviewed By: vsiles

Differential Revision: D40678341

fbshipit-source-id: d82d4353deead4998c7eb89a32d08fde300ec747

hphp/hack/.gitignore

@@ -19,6 +19,7 @@ facebook/opam2-mini-repository-*
 facebook/opam2-mini-repository-*.tar.gz
 target/
 dev_env.sh
+.merlin
 # Generated by scripts/build_rust_to_ocaml.sh; the script can't delete it
 # without creating a race condition when we're doing parallel builds.
 #