revbranchcache: add the necessary bit to send 'rbc' data over bundle2

Getbundle is now capable of sending rev-branch-cache information for the
changesets it bundle. The data sent are mostly nodes so it is quite compact.
The goal of the rev-branch-cache is to speed up branch map computation,
especially when the branchmap gets invalidated so we send data for all
exchanged changesets. In addition, computing the relevant heads to send in
case of partial pulling would be challenging.

The feature is still inactive since the capability is not advertised yet.

mercurial/exchange.py

@@ -1939,6 +1939,28 b' def _getbundletagsfnodes(bundler, repo, '
     outgoing = _computeoutgoing(repo, heads, common)
     bundle2.addparttagsfnodescache(repo, bundler, outgoing)
 
+@getbundle2partsgenerator('cache:rev-branch-cache')
+def _getbundlerevbranchcache(bundler, repo, source, bundlecaps=None,
+                             b2caps=None, heads=None, common=None,
+                             **kwargs):
+    """Transfer the rev-branch-cache mapping
+
+    The payload is a series of data related to each branch
+
+    1) branch name length
+    2) number of open heads
+    3) number of closed heads
+    4) open heads nodes
+    5) closed heads nodes
+    """
+    # Don't send unless:
+    # - changeset are being exchanged,
+    # - the client supports it.
+    if not (kwargs.get(r'cg', True)) or 'rev-branch-cache' not in b2caps:
+        return
+    outgoing = _computeoutgoing(repo, heads, common)
+    bundle2.addpartrevbranchcache(repo, bundler, outgoing)
+
 def check_heads(repo, their_heads, context):
     """check if the heads of a repo have been modified