fix: highlight the required configuration and behavior of the fixer tools The problem is that `hg help fix` didn't mention *how* to configure the tools, and while I knew that `{rootpath}` existed in the configuration, I missed that the tools require reading content from stdin. (I configured `gofmt` to use `{rootpath}`, and that had the effect of squashing all changes in a file at `.` into the first commit and emptying that content from its descendants.) Basically all this does is put a pointer in the default (command level) help to the extension level help that mentions the configuration, and moves the extension level help that documents reading from stdin and writing to stdout to the top to give it more prominence. The last sentence is adjusted a bit to reflect the new location.

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)

tests: in filterpyflakes, tolerate non-ascii file contents

comments: fix spelling

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.

chg: declare environ (issue6812)