dirstate: change placeholder hash length to 20 bytes

Previously the new-node placeholder hash for manifests generated from the
dirstate was a 21byte long string of "!" characters. Normal hashes are only 20
bytes long.  This makes it complicated to implement more efficient manifest
implementations which rely on the hashes being fixed length.

Let's change this hash to just be 20 bytes long, and rely on the astronomical
improbability of an actual hash being 20 "!" bytes in a row (just like we rely
on no hash ever being the nullid).

A future diff will do this for added and modified dirstate markers as well, so
we're putting the new newnodeid in node.py so there's a common place for these
placeholders.

mercurial/context.py

@@ -16,6 +16,7 b' from .i18n import _'
 from .node import (
     bin,
     hex,
+    newnodeid,
     nullid,
     nullrev,
     short,
@@ -39,11 +40,6 b' from . import ('
 
 propertycache = util.propertycache
 
-# Phony node value to stand-in for new files in some uses of
-# manifests. Manifests support 21-byte hashes for nodes which are
-# dirty in the working copy.
-_newnode = '!' * 21
-
 nonascii = re.compile(r'[^\x21-\x7f]').search
 
 class basectx(object):
@@ -142,7 +138,7 b' class basectx(object):'
                 removed.append(fn)
             elif flag1 != flag2:
                 modified.append(fn)
-            elif node2 != _newnode:
+            elif node2 != newnodeid:
                 # When comparing files between two commits, we save time by
                 # not comparing the file contents when the nodeids differ.
                 # Note that this means we incorrectly report a reverted change
@@ -1587,7 +1583,7 b' class workingctx(committablectx):'
         """
         mf = self._repo['.']._manifestmatches(match, s)
         for f in s.modified + s.added:
-            mf[f] = _newnode
+            mf[f] = newnodeid
             mf.setflag(f, self.flags(f))
         for f in s.removed:
             if f in mf:

mercurial/copies.py

@@ -278,7 +278,7 b' def _makegetfctx(ctx):'
         ac = repo.changelog.ancestors(revs, inclusive=True)
         ctx._ancestrycontext = ac
     def makectx(f, n):
-        if len(n) != 20:  # in a working context?
+        if len(n) != 20 or n in node.wdirnodes:  # in a working context?
             if ctx.rev() is None:
                 return ctx.filectx(f)
             return repo[None][f]

mercurial/node.py

@@ -17,6 +17,12 b' nullrev = -1'
 nullid = b"\0" * 20
 nullhex = hex(nullid)
 
+# Phony node value to stand-in for new files in some uses of
+# manifests.
+newnodeid = '!' * 20
+
+wdirnodes = set((newnodeid,))
+
 # pseudo identifiers for working directory
 # (they are experimental, so don't add too many dependencies on them)
 wdirrev = 0x7fffffff