testing: introduce util function to synchronize concurrent commands on files This is an extension of mechanisms that the tests have been using for a while. To be able to also control the execution in Rust, we introduce utility to perform such `wait_on_file` logic based on some configuration value. This will be used in the tests introduced in the next changesets.

hghave: drop py27 and py35 support These versions of python are no longer supported by Mercurial.

hghave: byteify a path passed to a core API It looks like this predicate isn't used at all(?)

dirstate: add some debug output when writing the dirstate This helps debugging Mercurial behavior in the dirstate-v2 case.

run-tests: make it possible to nest conditionals This is not that hard to implement and makes our life easier on a regular basis.

rust: box ConfigValueParseError to avoid large result types clippy emits a warning that all the Result types are way too large because of HgError includes ConfigValueParseError as one of the variants, so its size is 136 bytes. By boxing ConfigValueParseError we're hopefully making everything faster "for free".

dirstate: drop `identity` from the public API We no longer needs it.

delta-find: rename `delta-reuse-policy` to `pulled-delta-reuse-policy` This make it clearer which type of delta we are talking about.

config-item: declare undeclared path suboption This should prevent issue like the previous commit.

delta-find: declare the "paths..*:delta-reuse-policy option While looking into renaming the option I realized it was not declared.