revlog: introduce _chunkbase to allow filelog to override Used by revlog.revision to retrieve the base-chunk in a delta chain.

filelog: extract metadata parsing and packing _parsemeta returns the dictionary and a list of keys in the order they appear in metadata. This can be used to repack the dictionary in the same order. _packmeta creates metadata from a dictionary and an optional key-order list. In _parsemeta, we use slices and re.search indead of str.index so we can accept both buffers and strings.

discovery: drop findoutgoing and simplify findcommonincoming's api This is a long desired cleanup and paves the way for new discovery. To specify subsets for bundling changes, all code should use the heads of the desired subset ("heads") and the heads of the common subset ("common") to be excluded from the bundled set. These can be used revlog.findmissing instead of revlog.nodesbetween. This fixes an actual bug exposed by the change in test-bundle-r.t where we try to bundle a changeset while specifying that said changeset is to be assumed already present in the target. This used to still bundle the changeset. It no longer does. This is similar to the bugs fixed by the recent switch to heads/common for incoming/pull.

revset: add missing whitespace

url: remove unused/obsolete functions

revset: add ^ and ~ operators from parentrevspec extension ^ (Nth parent) and ~ (Nth first ancestor) are infix operators that match certain ancestors of the set: set^0 the set set^1 (also available as set^) the first parent of every changeset in set set^2 the second parent of every changeset in set set~0 the set set~1 the first ancestor (i.e. the first parent) of every changeset in set set~2 the second ancestor (i.e. first parent of first parent) of every changeset in set set~N the Nth ancestor (following first parents only) of every changeset in set; set~N is equivalent to set^1^1..., with ^1 repeated N times.

encoding: add an encoding-aware lower function