mq: when adding headers in plain mode, separate them from message (issue4453) c87f2a5a6e49 did a clean-up in one direction ... but we want it in the other direction.

mq: introduce insertplainheader - same naive implementation as before

mq: when setting message in plain mode, separate it from header (issue4453) Fix inconsistent handling of plain header separation in mq patcheader - and contrary to c87f2a5a6e49, do it in the direction of having an empty line between header and description. Plain patches are like mails and should thus have an empty line between headers and body in compliance with RFC 822 3.1.

setdiscovery: avoid a full changelog graph traversal We were definitely being suboptimal here: we were constructing two full sets, one with the full set of common nodes (i.e. a graph traversal) and one with all nodes. Then we subtract one set from the other. This whole process is O(commits) and causes discovery to be significantly slower than it should be. Instead, keep track of common incrementally and keep undecided as small as possible. This makes discovery massively faster on large repos: on one such repo, 'hg debugdiscovery' over SSH with one commit missing on the client and five on the server went from 4.5 seconds to 1.5. (An 'hg debugdiscovery' with no commits missing on the client, i.e. connection startup time, was 1.2 seconds.)

ancestor: add a way to remove ancestors of bases from a given set This and missingancestors can share state, which will turn out to be perfect for set discovery.

ancestor: add a way to add to bases of a missing ancestor object This will be useful for setdiscovery, since with that we incrementally add to our knowledge of common nodes.

ancestor: add a way to test whether a missing ancestor object has bases This is pretty trivial so there's no unit test coverage for it. This will be used by setdiscovery.

ancestor: remove now-unused missingancestors function Callers should use revlog.incrementalmissingrevs instead.

revlog: switch findmissing* methods to incrementalmissingrevs This will allow us to remove ancestor.missingancestors in an upcoming patch.

revlog: add a method to get missing revs incrementally This will turn out to be useful for discovery.