dagop: extract descendants() from revlog module This method needs to be implemented in other storage backends and is generic if we parameterize the functions for retrieving revision numbers and parent revision numbers. Let's extract it to dagop so backends don't need to reinvent the wheel. I'm not too worried about performance overhead of the extra function call because I don't expect anyone to be calling descendants() in a tight loop. Differential Revision: https://phab.mercurial-scm.org/D4794

filelog: remove checkhash() (API) It is unused. While a caller may want to ask the store to validate the hash of some provided text, since there are no in-core consumers of this method, let's drop it for now. Differential Revision: https://phab.mercurial-scm.org/D4793

filelog: remove revdiff() (API) This proxy method is no longer used. While it might be useful to query a storage backend for the delta between any 2 revisions because the store could have a delta cached and could compute it more efficiently than the caller calling revision() twice in order to compute a delta, since nothing in core is using this API now, I feel comfortable nuking it. Differential Revision: https://phab.mercurial-scm.org/D4792

localrepo: define storage backend in creation options (API) We add an experimental config option to define the storage backend for new repositories. By default, it uses "revlogv1," which maps to the current and only modern supported repository format. We add a "backend" creation option to control which backend to use. It defaults to using the value from the config option. newreporequirements() will now barf if it sees a "backend" value that isn't "revlogv1." This forces extensions to monkeypatch the function to handle requirements derivation for custom backends. In order for this to "just work," we factored out obtaining the default creation options into its own function and made callers of newreporequirements() responsible for passing in valid data. Without this, direct callers of newreporequirements() wouldn't get the proper results. Differential Revision: https://phab.mercurial-scm.org/D4791

wireprotov2: derive "required" from presence of default value If we define a default value for all optional arguments, we no longer need to explicitly declare whether the argument is required. Instead, we can derive it from the presence of a default value callable. Differential Revision: https://phab.mercurial-scm.org/D4790

localrepo: capture repo interface factory functions as lambas Previously, the list would hold a reference to the original function and attempting to wrap the module function via extension wouldn't result in the wrapped function being called, completely undermining the expected behavior. Differential Revision: https://phab.mercurial-scm.org/D4821

extensions: new closehead module for closing arbitrary heads ``hg close-head`` allows closing arbitrary heads. It is equivalent to checking out the given revisions and commit an empty change with ``hg commit --close-branch``. Differential Revision: https://phab.mercurial-scm.org/D3557