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/support/test_helper.sh

@@ -34,7 +34,7 @@ verboseGitCommands() {
 #-----------------------------------------------------------------------------
 
 _includes() {
-  if [ -n "$3" ]; then regex="$3"; else regex=P; fi
+  if [ -n "$3" ]; then regex="$3"; else regex=''; fi
   if echo "$1" | grep -q$regex "$2"; then echo 0; else echo 1; fi
 }
 
@@ -46,4 +46,3 @@ assertIncludes() {
 assertNotIncludes() {
   assertFalse "'$1' should not have contained '$2'" $(_includes "$@")
 }
-