Added a controls_location implementation to the Mercurial class, …

…so it porperly detects that subdirectories are under mercurial control.

src/pip/_internal/vcs/mercurial.py

@@ -9,6 +9,7 @@
 from pip._internal.utils.temp_dir import TempDirectory
 from pip._internal.utils.typing import MYPY_CHECK_RUNNING
 from pip._internal.vcs.versioncontrol import VersionControl, vcs
+from pip._internal.exceptions import BadCommand
 
 if MYPY_CHECK_RUNNING:
     from pip._internal.utils.misc import HiddenText
@@ -111,5 +112,16 @@ def is_commit_id_equal(cls, dest, name):
         """Always assume the versions don't match"""
         return False
 
+    @classmethod
+    def controls_location(cls, location):
+        if super(Mercurial, cls).controls_location(location):
+            return True
+        try:
+            r = cls.run_command(['identify'], cwd=location, show_stdout=False, extra_ok_returncodes=[255])
+            return not r.startswith('abort:')
+        except BadCommand:
+            logger.debug("could not determine if %s is under hg control "
+                         "because hg is not available", location)
+            return False
 
 vcs.register(Mercurial)