perf: update function name to match `perfbranchmapload` command Having function with the same name as the command is simpler.

perf: fix a minor typo in perfbranchmapload

context: floor adjustlinkrev graph walk during copy tracing The `_adjustlinkrev` method gains an optional "stoprev" argument. The linkrev adjustment will give up once this floor is reached. The relevant functions using `_adjustlinkrev` are updated to pass an appropriate value in the copy tracing code. In some private repository, about 10% of the status call triggered the pathological case addressed by this change. The speedup varies from one call to another, the best-observed win is moving from 170s to 11s. The effect of this change can be seen in the public pypy repository, running the following command: hg perftracecopies --source 83c9ff0c0206 --destination 59c79103d5b0 before: 3.401753 seconds after: 2.634897 seconds (-23%)

context: small refactoring of `isintroducedafter` This make the next change clearer.

context: split `introrev` logic in a sub function We want to add a mechanism to stop iteration early associated to intro rev early in some case. However, it does not make sense to expose it in the public `filectx` API. So we split the code into an internal method instead.

context: introduce an `isintroducedafter` method and use it in copies Right now, copy tracing make effort to not traverse the graph too much to save performance. It uses a "limit" acting as a floor revision past which data are no longer relevant to the current copy tracing. However, to enforce this limit, it does a call to `filectx.rev()` and that call can trigger a graph traversal on its own. That extra graph traversal is unaware of the current limit and can become very expensive. That cost is increased by the nature of work done in adjust link rev, we are not only walking down the graph, we are also checking the affected file for each revision we walk through. Something significantly more expensive than the walk itself. To work around this we need to make the `filectx` operation aware of the current limit. The first step is to introduce a dedicated method: `isintroducedafter`. We'll then rework that method logic to stop traversal as soon as possible.

context: take advantage of `_descendantrev` in introrev if available Before this changeset, `_descendantrev` was ignored and `introrev` could return a "wrong" result. I was previously fine because there seems to be no existing code using both `introrev` and `_descendantrev` at the same time. However, we would like to change that.

context: spell out the logic around linkrev adjustement starting point We make the intent of the `_changeid` and `_changectx` checks explicit. The same logic was previously performed by the `self.rev()` call. The new code is a bit redundant, but much clearer.

perf: add a new `perfhelper-tracecopies` command The command is not measuring performance itself, it digs interesting statistic to help pick good arguments for the `perfcopytrace` command.

narrow: extract helper for parsing narrowspec file This will be used for parsing a file that's stored in a different location. Differential Revision: https://phab.mercurial-scm.org/D5277