scmutil: introduce filterrequirements() to split reqs into wc and store ones

scmutil: introduce filterrequirements() to split reqs into wc and store ones

In upcoming patches where we try to implement requirements in store, we will
need a mechanism to split all requirements on some basis and decide which one
goes to `.hg/requires` and which one goes to `.hg/store/requires`.

This patch introduce a separate function for that. Filtering logic for now is
put under an `if False:`. In upcoming patches it will be removed.

Differential Revision: https://phab.mercurial-scm.org/D8913

--- a/mercurial/scmutil.py	Thu Sep 17 18:28:53 2020 -0700
+++ b/mercurial/scmutil.py	Fri Aug 07 16:11:19 2020 +0530
@@ -38,6 +38,7 @@
     phases,
     policy,
     pycompat,
+    requirements as requirementsmod,
     revsetlang,
     similar,
     smartset,
@@ -1470,11 +1471,34 @@
     repo._quick_access_changeid_invalidate()
 
 
+def filterrequirements(requirements):
+    """ filters the requirements into two sets:
+
+    wcreq: requirements which should be written in .hg/requires
+    storereq: which should be written in .hg/store/requires
+
+    Returns (wcreq, storereq)
+    """
+    if False:
+        wc, store = set(), set()
+        for r in requirements:
+            if r in requirementsmod.WORKING_DIR_REQUIREMENTS:
+                wc.add(r)
+            else:
+                store.add(r)
+        return wc, store
+    return requirements, None
+
+
 def writereporequirements(repo, requirements=None):
     """ writes requirements for the repo to .hg/requires """
     if requirements:
         repo.requirements = requirements
-    writerequires(repo.vfs, repo.requirements)
+    wcreq, storereq = filterrequirements(repo.requirements)
+    if wcreq is not None:
+        writerequires(repo.vfs, wcreq)
+    if storereq is not None:
+        writerequires(repo.svfs, storereq)
 
 
 def writerequires(opener, requirements):