storageutil: extract filelog.cmp() to a standalone function

storageutil: extract filelog.cmp() to a standalone function

As part of implementing an alternate storage backend, I found myself
reimplementing this code.

With a little massaging, we can extract filelog.cmp() to a standalone
function.

As part of this, the call to revlog.cmp() was inlined (it is just a
2-line function).

I also tweaked some variable names to improve readability. I'll
further tweak names in a subsequent commit.

Differential Revision: https://phab.mercurial-scm.org/D4801

--- a/mercurial/filelog.py	Fri Sep 28 11:37:49 2018 -0700
+++ b/mercurial/filelog.py	Fri Sep 28 11:47:53 2018 -0700
@@ -135,26 +135,7 @@
 
         returns True if text is different than what is stored.
         """
-
-        t = text
-        if text.startswith('\1\n'):
-            t = '\1\n\1\n' + text
-
-        samehashes = not self._revlog.cmp(node, t)
-        if samehashes:
-            return False
-
-        # censored files compare against the empty file
-        if self.iscensored(self.rev(node)):
-            return text != ''
-
-        # renaming a file produces a different hash, even if the data
-        # remains unchanged. Check if it's the case (slow):
-        if self.renamed(node):
-            t2 = self.read(node)
-            return t2 != text
-
-        return True
+        return storageutil.filerevisiondifferent(self, node, text)
 
     def verifyintegrity(self, state):
         return self._revlog.verifyintegrity(state)

--- a/mercurial/utils/storageutil.py	Fri Sep 28 11:37:49 2018 -0700
+++ b/mercurial/utils/storageutil.py	Fri Sep 28 11:47:53 2018 -0700
@@ -108,6 +108,32 @@
 
     return False
 
+def filerevisiondifferent(store, node, filedata):
+    """Determines whether file data is equivalent to a stored node."""
+
+    if filedata.startswith(b'\x01\n'):
+        revisiontext = b'\x01\n\x01\n' + filedata
+    else:
+        revisiontext = filedata
+
+    p1, p2 = store.parents(node)
+
+    computednode = hashrevisionsha1(revisiontext, p1, p2)
+
+    if computednode == node:
+        return False
+
+    # Censored files compare against the empty file.
+    if store.iscensored(store.rev(node)):
+        return filedata != b''
+
+    # Renaming a file produces a different hash, even if the data
+    # remains unchanged. Check if that's the case.
+    if store.renamed(node):
+        return store.read(node) != filedata
+
+    return True
+
 def iterrevs(storelen, start=0, stop=None):
     """Iterate over revision numbers in a store."""
     step = 1