http: added service backend for testing communication, and extracting vcsserver version via remote calls.

backends: implemented functions for fetching backend versions via remote calls. Ref: #4276

subrepo: Apply mercurial sub repository patch.

subrepo: Turn off interactive mode when merging mercurial repo. If there is a merge conflict in the sub repositories mercurial will prompt the used to decide which version to use.

subrepo: Add custom exception to indicate subrepo merge conflict.

subrepo: Add patch to turn off mercurial subrepo handling.

requirements: added ipdb (0.10.1) support.

nix: Move shell hook from default.nix -> shell.nix and set custom shell prompt.

release: merge back stable branch into default

Added tag v4.4.2 for changeset 24ffe44a27fc