Add test for file moves

See #872.

test/test_gitgutter.vim

@@ -52,6 +52,7 @@ endfunction
 "
 
 function SetUp()
+  let g:gitgutter_diff_base = ''
   call system("git init ".s:test_repo.
         \ " && cd ".s:test_repo.
         \ " && cp ../.gitconfig .".
@@ -280,6 +281,29 @@ function Test_saveas()
 endfunction
 
 
+function Test_file_mv()
+  call system('git mv fixture.txt fixture_moved.txt')
+  edit fixture_moved.txt
+  normal ggo*
+  call s:trigger_gitgutter()
+  let expected = [{'lnum': 2, 'name': 'GitGutterLineAdded'}]
+  call s:assert_signs(expected, 'fixture_moved.txt')
+
+  write
+  call system('git add fixture_moved.txt && git commit -m "moved and edited"')
+  GitGutterDisable
+  GitGutterEnable
+  let expected = []
+  call s:assert_signs(expected, 'fixture_moved.txt')
+
+  GitGutterDisable
+  let g:gitgutter_diff_base = 'HEAD^'
+  GitGutterEnable
+  let expected = [{'lnum': 2, 'name': 'GitGutterLineAdded'}]
+  call s:assert_signs(expected, 'fixture_moved.txt')
+endfunction
+
+
 " FIXME: this test fails when it is the first (or only) test to be run
 function Test_follow_symlink()
   let tmp = 'symlink'