requirements: introduce a set of working directory specific requirements

Some requirements like the SPARSE_REQUIREMENT is working directory specific and
cannot be shared. We add a set which will contain all these requirements.

This is not the best we can do, I think having a rich requirement class will be
much better but that will be out of scope for this series.

Differential Revision: https://phab.mercurial-scm.org/D8924

mercurial/requirements.py

@@ -44,3 +44,8 b" COPIESSDC_REQUIREMENT = b'exp-copies-sid"
 
 # The repository use persistent nodemap for the changelog and the manifest.
 NODEMAP_REQUIREMENT = b'persistent-nodemap'
+
+# List of requirements which are working directory specific
+# These requirements cannot be shared between repositories if they
+# share the same store
+WORKING_DIR_REQUIREMENTS = {SPARSE_REQUIREMENT}