merge: change modified indicator to be 20 bytes

Previously we indicated that the .hgsubstate file was dirty by adding a '+' to
the end of its hash in the wctx manifest. This made is complicated to have new
manifest implementations that rely on the node length being fixed.

In previous patches we added added and modified node placeholders, so let's use
those to indicate dirty here as well. It doesn't look like anything ever
depended on this '+' (aside from it being different to the parent), so nothing
else needed to change here.

mercurial/merge.py

@@ -18,6 +18,7 b' from .node import ('
     addednodeid,
     bin,
     hex,
+    modifiednodeid,
     nullhex,
     nullid,
     nullrev,
@@ -815,7 +816,7 b' def manifestmerge(repo, wctx, p2, pa, br'
     if '.hgsubstate' in m1:
         # check whether sub state is modified
         if any(wctx.sub(s).dirty() for s in wctx.substate):
-            m1['.hgsubstate'] += '+'
+            m1['.hgsubstate'] = modifiednodeid
 
     # Compare manifests
     if matcher is not None: