Updated Rust wrapper so that hg status works again in Eden.

Summary:
The storage format for dirstate data in Eden changed substantially in D6179950.
Although `status.rs` was modified as part of that change, it was only updated to
make it so the rust wrapper still compiled, but it did not maintain the
correctness of the overall function.

This revision is a port of the logic in `eden/py/dirstate.py` and
`eden/hg/eden/EdenThriftClient.py` so that the Python code in the Eden/Hg
extension behaves the same as that of the Rust wrapper.

The current implementation of `hg status` in Rust does not relativize paths to `$PWD`, so
it always behaves as if `--root-relative` was specified. This should be fixed in a
follow-up revision.

Reviewed By: jsgf

Differential Revision: D6717211

fbshipit-source-id: ca900e251d392a77c07cb7305c4d4a62def1c7ab

eden/integration/lib/TARGETS

@@ -5,6 +5,8 @@ python_library(
         "//eden/cli:lib",
         "//eden/fs/service:py-client",
         "//eden/fs/service:thrift-py",
+        "//libfb/py:pathutils",
+        "//scm/telemetry/hg:hg",
     ],
     external_deps = [
         "py-hypothesis",

eden/integration/lib/hgrepo.py

@@ -63,8 +63,7 @@ class HgRepository(repobase.Repository):
         # Set HGRCPATH to make sure we aren't affected by the local system's
         # mercurial settings from /etc/mercurial/
         self.hg_environment['HGRCPATH'] = ''
-        self.hg_bin = distutils.spawn.find_executable(
-            'hg.real') or distutils.spawn.find_executable('hg')
+        self.hg_bin = find_hg_bin()
 
     def hg(
         self,
@@ -215,3 +214,35 @@ class HgRepository(repobase.Repository):
         else:
             args = ['reset', rev]
         self.hg(*args, stdout=None, stderr=None)
+
+
+_hg_bin = None
+
+
+def find_hg_bin() -> str:
+    global _hg_bin
+    if _hg_bin:
+        return _hg_bin
+
+    try:
+        from libfb.py import pathutils
+        _hg_bin = pathutils.get_build_rule_output_path(
+            '@/scm/telemetry/hg:hg',
+            pathutils.BuildRuleTypes.RUST_BINARY,
+            start_path=__file__
+        )
+        if _hg_bin:
+            return _hg_bin
+    except ImportError:
+        # Code to load the custom Hg wrapper was not available.
+        pass
+
+    _hg_bin = distutils.spawn.find_executable('hg.real')
+    if _hg_bin:
+        return _hg_bin
+
+    _hg_bin = distutils.spawn.find_executable('hg')
+    if _hg_bin:
+        return _hg_bin
+
+    raise Exception('No hg binary found!')