Add a simple test in geef_oid

Makefile

@@ -13,3 +13,6 @@ compile: $(REBAR)
 
 clean: $(REBAR)
 	@$(REBAR) clean
+
+test: $(REBAR)
+	@$(REBAR) eunit

src/geef_oid.erl

@@ -1,6 +1,11 @@
 -module(geef_oid).
 
+-ifdef(TEST).
+-include_lib("eunit/include/eunit.hrl").
+-endif.
+
 -include("geef_records.hrl").
+
 -export([parse/1, fmt/1]).
 
 -spec fmt(oid()) -> binary().
@@ -12,3 +17,13 @@ fmt(#oid{oid=Oid}) ->
 parse(Sha) ->
     Oid = geef:oid_parse(Sha),
     #oid{oid=Oid}.
+
+-ifdef(TEST).
+
+back_and_forth_test() ->
+    ?assertMatch(<<"d71c6ff702e75247ce29c51279d78a7a202f5cc9">>,
+		 fmt(parse(<<"d71c6ff702e75247ce29c51279d78a7a202f5cc9">>))),
+    ?assertMatch(<<"d71c6ff702e75247ce29c51279d78a7a202f5cc9">>,
+		 fmt(parse("d71c6ff702e75247ce29c51279d78a7a202f5cc9"))).
+
+-endif.