Improve the performance of v2 changed. (pantsbuild#5571)

this is good for a ~40x speedup in my test case:

before:

[omerta pants (kwlzn/faster_v2_changed)]$ time ./pants --changed-parent=HEAD~60 list > a

real    4m26.282s
user    2m59.511s
sys     1m25.936s
after:

[omerta pants (kwlzn/faster_v2_changed)]$ time ./pants --changed-parent=HEAD~60 list > b

real    0m7.668s
user    0m6.251s
sys     0m1.425s

Fixes pantsbuild#5570

src/python/pants/engine/legacy/BUILD

@@ -77,6 +77,7 @@ python_library(
     'src/python/pants/base:specs',
     'src/python/pants/build_graph',
     'src/python/pants/source',
+    '3rdparty/python:six'
   ]
 )
 

src/python/pants/engine/legacy/source_mapper.py

@@ -7,6 +7,8 @@
 
 import os
 
+import six
+
 from pants.base.specs import AscendantAddresses, SingleAddress
 from pants.build_graph.address import parse_spec
 from pants.build_graph.source_mapper import SourceMapper
@@ -75,13 +77,15 @@ def iter_target_addresses_for_sources(self, sources):
     sources_set = set(sources)
     subjects = [AscendantAddresses(directory=d) for d in self._unique_dirs_for_sources(sources_set)]
 
+    # Uniqify all transitive hydrated targets.
+    hydrated_target_to_address = {}
     for hydrated_targets in self._scheduler.product_request(HydratedTargets, subjects):
       for hydrated_target in hydrated_targets.dependencies:
-        legacy_address = hydrated_target.adaptor.address
-
-        # Handle BUILD files.
-        if any(LegacyAddressMapper.is_declaring_file(legacy_address, f) for f in sources_set):
-          yield legacy_address
-        else:
-          if any(self._owns_source(source, hydrated_target) for source in sources_set):
-            yield legacy_address
+        if hydrated_target not in hydrated_target_to_address:
+          hydrated_target_to_address[hydrated_target] = hydrated_target.adaptor.address
+
+    for hydrated_target, legacy_address in six.iteritems(hydrated_target_to_address):
+      # Handle BUILD files.
+      if (any(LegacyAddressMapper.is_declaring_file(legacy_address, f) for f in sources_set) or
+          any(self._owns_source(source, hydrated_target) for source in sources_set)):
+        yield legacy_address

src/python/pants/source/wrapped_globs.py

@@ -13,7 +13,7 @@
 from twitter.common.dirutil.fileset import Fileset
 
 from pants.base.build_environment import get_buildroot
-from pants.util.dirutil import fast_relpath
+from pants.util.dirutil import fast_relpath, fast_relpath_optional
 from pants.util.memo import memoized_property
 from pants.util.meta import AbstractClass
 
@@ -104,8 +104,8 @@ def __repr__(self):
     return 'EagerFilesetWithSpec(rel_root={!r}, files={!r})'.format(self.rel_root, self.files)
 
   def matches(self, path_from_buildroot):
-    path_relative_to_rel_root = os.path.relpath(path_from_buildroot, self.rel_root)
-    return path_relative_to_rel_root in self._files
+    path_relative_to_rel_root = fast_relpath_optional(path_from_buildroot, self.rel_root)
+    return path_relative_to_rel_root is not None and path_relative_to_rel_root in self._files
 
 
 class LazyFilesetWithSpec(FilesetWithSpec):