sparse: variable to track if sparse is enabled

Currently, the sparse extension sniffs repo instances for
attributes defined by the sparse extension to determine if
sparse is enabled. As we move code away from repo instances,
these checks will be a bit more brittle.

We introduce a module-level variable to track whether sparse is
enabled as a temporary workaround.

hgext/sparse.py

@@ -110,6 +110,8 b' def uisetup(ui):'
     _setupcommit(ui)
 
 def extsetup(ui):
+    sparse.enabled = True
+
     _setupclone(ui)
     _setuplog(ui)
     _setupadd(ui)

mercurial/sparse.py

@@ -12,6 +12,11 b' from . import ('
     error,
 )
 
+# Whether sparse features are enabled. This variable is intended to be
+# temporary to facilitate porting sparse to core. It should eventually be
+# a per-repo option, possibly a repo requirement.
+enabled = False
+
 def parseconfig(ui, raw):
     """Parse sparse config file content.