changegroup: fix to allow empty manifest parts The current chunk reading algorithm relied on counting the number of empty chunks and comparing it to the number of chunk lists it expected (1 list of files for cg1 and cg2, and 1 list of files + 1 list of trees for cg3). This implicitly assumed that both the changelog part and the manifestlog part were never empty (since them being empty would cause it to count it as one list being done, and screw up the count). In our treemanifest code, the manifest section could be empty, so we need to handle that case. This patches refactors that code to be more explicit about how it counts the expected parts. Differential Revision: https://phab.mercurial-scm.org/D646