changegroup: define functions for creating changegroup packers

Currently, we have 3 classes for changegroup generation. Each class
handles a specific changegroup format. And each subsequent version's
class inherits from the previous one.

The interface for the classes is not very well defined and a lot of
version-specific behavior is behind overloaded functions. This
approach adds complexity and makes changegroup generation difficult
to reason about.

Upcoming commits will be consolidating these 3 classes so differences
between changegroup versions and changegroup generation are controlled
by parameters to a single constructor / type rather than by
overriding class attributes via inheritance.

We begin this process by building dedicated functions for creating
each changegroup packer instance. Currently they just call the
constructor on the appropriate class. This will soon change.

Differential Revision: https://phab.mercurial-scm.org/D4076

mercurial/changegroup.py

@@ -1176,11 +1176,20 b' class cg3packer(cg2packer):'
         return struct.pack(
             self.deltaheader, node, p1n, p2n, basenode, linknode, flags)
 
-_packermap = {'01': (cg1packer, cg1unpacker),
+def _makecg1packer(repo, filematcher, bundlecaps):
+    return cg1packer(repo, filematcher, bundlecaps=bundlecaps)
+
+def _makecg2packer(repo, filematcher, bundlecaps):
+    return cg2packer(repo, filematcher, bundlecaps=bundlecaps)
+
+def _makecg3packer(repo, filematcher, bundlecaps):
+    return cg3packer(repo, filematcher, bundlecaps=bundlecaps)
+
+_packermap = {'01': (_makecg1packer, cg1unpacker),
              # cg2 adds support for exchanging generaldelta
-             '02': (cg2packer, cg2unpacker),
+             '02': (_makecg2packer, cg2unpacker),
              # cg3 adds support for exchanging revlog flags and treemanifests
-             '03': (cg3packer, cg3unpacker),
+             '03': (_makecg3packer, cg3unpacker),
 }
 
 def allsupportedversions(repo):
@@ -1249,8 +1258,8 b' def getbundler(version, repo, bundlecaps'
     filematcher = matchmod.intersectmatchers(repo.narrowmatch(),
                                              filematcher)
 
-    return _packermap[version][0](repo, filematcher=filematcher,
-                                  bundlecaps=bundlecaps)
+    fn = _packermap[version][0]
+    return fn(repo, filematcher, bundlecaps)
 
 def getunbundler(version, fh, alg, extras=None):
     return _packermap[version][1](fh, alg, extras=extras)