dont use grep -P for cross-platform compatibility

`grep -P` gives access to the PCRE matching engine, which is nice,
however, the version of grep shipped on many BSD systems (including
Darwin) does not have this flag.

Currently, `grep -P` was being used in the `_includes()` test helper.

For cross platform compatibility in tests, do not rely upon this option.
Luckily, all existing tests seem to work fine without it already!

test/lib/git/shell_shortcuts_test.sh

@@ -82,7 +82,7 @@ test_ls_with_file_shortcuts() {
   ls_with_file_shortcuts > $temp_file
   ls_output=$(<$temp_file strip_colors)
 
-  # Compare as fixed strings (F), instead of regex (P)
+  # Compare as fixed strings (F), instead of normal grep behavior
   assertIncludes "$ls_output" '[1]  a "b"' F
   assertIncludes "$ls_output" "[2]  a 'b'" F
   assertIncludes "$ls_output" '[3]  a [b]' F