rhg: make `rhg files` work if `ui.relative-files=true` is specified

rhg: test `rhg files --config ui.relative-paths ...` Add some tests. All of these are falling back for now, will be fixed in follow-up commits.

cleanup: simplify code

dirstate: better error messages when dirstate is corrupted The current error message "Overflow in dirstate" sounds confusing because it suggests either a certain size limit that's being exceeded, or integer arithmetic overflowing. The reality is just a file being shorter than expected.

rust: remove an unused error variant DirstateMapError::EmptyPath

hg: move unreachable code to where it could be reached

stream-clone: support streamv3 on the cli [hg bundle] We add support and test for this. The support is still experimental, so the various name and identifier will eventually need to be renamed when stream-v3 gets out of experimental.

stream-clone: add the `-exp` prefix to the bundle part We forget to do so in 58adcabc295f, however this is important to prevent "current" client to send incompatible version to future client.

zstd: hack include order to ensure that our zstd.h is found If the regular Python CFLAGS include directories that already have the zstd headers available, a different and possible incompatible version can be picked up otherwise. Sadly, it seems like Python has no easy way to prefix flags before the rest.

exchange: allow passing no includes/excludes to `pull()` I would expect that `exchange.pull(includepats=[])` results in an empty list of include patterns to be passed to the remote, but it doesn't currently because we check for any truthy value instead of checking specifically for `not None`.