help: move Windows 9x information to appropriate place

shelve: move commitfunc closer to use site Supporting shelving of unknown files needs this code motion. No functional changes.

commands: document clone bundles hooks and rollback behavior The added content is inside a verbose container. I figure it makes sense to explicitly document behavior, including with the caveat it may change later. People can't say they weren't warned!

clonebundles: improve BUNDLESPEC documentation Before the existence of `hg debugbundle --spec`, the process for defining the BUNDLESPEC value in manifests was not very clear and not trivial to automate, especially in the case of stream clone bundles. This patch adds documentation to note the existence of `hg debugbundle --spec`. We drop the reference to stream clone requirements handling because it is now redundant with `hg debugbundle --spec`. While we are here, we further reinforce the importance of defining BUNDLESPEC.

commands: teach debugbundle to print bundle specification This seems like the most logical place to put this functionality. Test coverage over existing known bundle specs has been added.

commands: use context manager for opened bundle file

exchange: implement function for inferring bundle specification We don't currently have a mechanism for inferring bundle spec strings from bundle files. This patch adds one. This will eventually be used to make the producing of clone bundles manifests easier.

streamclone: extract code for reading header fields So it can be called from another consumer in a future patch.

encoding: escape U+007F (DEL) character in JSON RFC 7159 does not state that U+007F must be escaped, but it is widely considered a control character. As '\x7f' is invisible on a terminal, and Python's json.dumps() escapes '\x7f', let's do the same.

run-tests: skip threading for a single test (issue5040) This version backs out 50e621fe0362 and implements it in a more consistent manner.