debug-delta: add minimal documentation for `devel.bundle-delta` option So the next person won't have to grep the code to know what it is doing.

delta-find: simply code that is now never invoqued without general delta Finish early in on-general delta case mean a lot of code can assume that general delta is not enabled.

delta-find: add a simple safeguard to prevent bad non-general-delta This make it easier to catch request to build delta that does not make sense in the general delta world. It seems better to raise a programming error than to corrupt the repository.

delta-find: never do anything fancy when general delta is off Without general delta, the only option is to do a delta against the previous revision in the revlog, or to do a full snapshot. So no need to dive into the more complicated logic. This prevent Mercurial to misbehave when applying remote deltas when `pulled-delta-reuse-policy` was set to "try-base" or "forced" and would produce and store invalid deltas. We add an associated tests to make sure this scenario is covered by tests. There are more code to cleanup after this, but I what to keep the fixing patch simple.

delta-find: assume the target-rev if not specified This will be useful in the next patch, but I am putting it in its own commit to make sure we can easily bisect any subtle side effect. (none are expected, but still)

chg: set CHGHG before connecting to command server cf4d2f31 (!523) changed chg to set `CHGHG` itself when spawning a new command server, in order to ensure that the path to the `hg` executable would be checked during server validation. (This is useful when chg is built with `HGPATHREL`). However, that change broke chg because it failed to set `CHGHG` before trying to connect to an existing command server. This means that if `CHGHG` is not present in the environment, chg will always spawn a new command server, entirely negating the point of chg. This breakage wasn't initially caught because of the difficulty of writing automated tests with the `HGPATHREL` feature enabled, which meant the change was only tested manually to make sure that it fixed the problem with `HGPATHREL` that prompted the change. In practice, this functionality is only really useful when chg is built with `HGPATHREL`, so I considered wrapping it in an `#ifdef` to preserve the old behavior by default. However, this makes it hard to write tests since one would have to explicitly set `HGPATHREL=1` when running `run-tests.py` (which is why the original change lacked tests). It would be great if there were a way of testing features that are gated behind conditional compilation.