storageutil: extract revision number iteration

This code is a bit quirky (and possibly buggy). It will likely be used
by multiple storage backends. Let's extract it so it is reusable.

Differential Revision: https://phab.mercurial-scm.org/D4757

mercurial/revlog.py

@@ -515,17 +515,7 b' class revlog(object):'
         return iter(pycompat.xrange(len(self)))
     def revs(self, start=0, stop=None):
         """iterate over all rev in this revlog (from start to stop)"""
-        step = 1
-        length = len(self)
-        if stop is not None:
-            if start > stop:
-                step = -1
-            stop += step
-            if stop > length:
-                stop = length
-        else:
-            stop = length
-        return pycompat.xrange(start, stop, step)
+        return storageutil.iterrevs(len(self), start=start, stop=stop)
 
     @util.propertycache
     def nodemap(self):

mercurial/utils/storageutil.py

@@ -13,6 +13,9 b' import re'
 from ..node import (
     nullid,
 )
+from .. import (
+    pycompat,
+)
 
 _nullhash = hashlib.sha1(nullid)
 
@@ -81,3 +84,18 b' def filtermetadata(text):'
 
     offset = text.index(b'\x01\n', 2)
     return text[offset + 2:]
+
+def iterrevs(storelen, start=0, stop=None):
+    """Iterate over revision numbers in a store."""
+    step = 1
+
+    if stop is not None:
+        if start > stop:
+            step = -1
+        stop += step
+        if stop > storelen:
+            stop = storelen
+    else:
+        stop = storelen
+
+    return pycompat.xrange(start, stop, step)