bundle2.unbundlepart: decouple mandatory from parttype Encoding whether or not a part is mandatory in the capitalization of the parttype is unintuitive and error-prone. This sequence of patches separates these concerns in the API to reduce programmer error and pave the way for a potential change in how this information is transmitted over the wire. This patch separates the two pieces of information when reading the part header so that it's unnecessary to know how they were combined during transmission.

memctx: fix manifest for removed files (issue4470) filectxfn returns None for removed files, so we have to check for None before computing the new file content hash for the manifest. Includes a test that proves this works, by demonstrating that we can show the diff of an amended commit in the committemplate.