[#18] Added git staged files test case.

Author: jfacorro

test/elvis.coverspec

@@ -6,6 +6,7 @@
   elvis_result,
   elvis_utils,
   elvis_style,
+  elvis_project,
   elvis_config,
   elvis_github,
   elvis_git,

test/git_SUITE.erl

@@ -5,7 +5,8 @@
         ]).
 
 -export([
-         relative_position_from_patch/1
+         relative_position_from_patch/1,
+         check_staged_files/1
         ]).
 
 -define(EXCLUDED_FUNS,
@@ -61,3 +62,15 @@ relative_position_from_patch(_Config) ->
 
     not_found = elvis_git:relative_position(Patch, 174),
     not_found = elvis_git:relative_position(Patch, 180).
+
+
+-spec check_staged_files(config()) -> any().
+check_staged_files(_Config) ->
+    Filename = "../../temp_file_test",
+    file:write_file(Filename, <<"sdsds">>, [append]),
+
+    os:cmd("git add " ++ Filename),
+    [#{path := "temp_file_test"}] = elvis_git:staged_files(),
+    os:cmd("git reset " ++ Filename),
+
+    file:delete(Filename).