errors: add config that lets user get more detailed exit codes This adds an experimental config that lets the user get more detailed exit codes. For example, there will be a specific error code for input/user errors. This is part of https://www.mercurial-scm.org/wiki/ErrorCategoriesPlan. I've made the config part of tweakdefaults. I've made the config enabled by default in tests. My reasoning is that we want to see that each specific error case gives the right exit code and we don't want to duplicate all error cases in the entire test suite. It also makes it easy to grep the `.t` files for `[255]` to find which cases we have left to fix. The logic for the current exit codes is quite simple, so I'm not too worried about regressions there. I've added a test case specifically for the "legacy" exit codes. I've set the detailed exit status only for the case of `InterventionRequired` and `SystemExit` for now (the cases where we currently return something other than 255), just to show that it works. Differential Revision: https://phab.mercurial-scm.org/D9238

worker: raise exception instead of calling sys.exit() with child's code When a worker process returns an error code, we would call `sys.exit()` with that exit code on the main process. The `SystemExit` exception would then get caught in `scmutil.callcatch()`, which would return that error code. The comment there says "Commands shouldn't sys.exit directly", which I agree with. This patch changes it so we raise a specific exception when a worker fails so we can catch instead. I think that means that `SystemExit` is now always an internal error. (I had earlier thought that this call to `sys.exit()` was from within the child process until Matt Harbison made me look again, so thanks for that!) Differential Revision: https://phab.mercurial-scm.org/D9287

config: read system hgrc in lexicographical order This is similar to edbcf5b239f9 (config: read configs from directories in lexicographical order, 2019-04-03). Apparently I forgot to sort the system hgrc files there. That's fixed by this patch. Differential Revision: https://phab.mercurial-scm.org/D9269

relnotes: drop 5.6 release entries from next Differential Revision: https://phab.mercurial-scm.org/D9282

merge with stable

makefile: use Python 3 by default (BC) This change is long overdue IMO. .. bc:: Makefile now uses `python3` instead of `python` by default on non-Windows platforms. This means Mercurial will be built and run with Python 3 instead of Python 2.7 by default. To continue using Python 2, set the PYTHON variable. e.g. `make install PYTHON=python2.7`. Differential Revision: https://phab.mercurial-scm.org/D7258

hgweb: don't call sys.exit() in httpservice.run() If I'm reading the code correctly, `mercurial.server.createservice()` can return an hgweb service or one of three types of command server services. The caller then calls `mercurial.server.runservice()`, passing it the returned service's run method. Only the hgweb service was calling `sys.exit()`. It has been that way since 8d44649df03b (refactor ssh server., 2006-06-04). That commit message doesn't provide any explanation. Let's clean up and have the code follow the usual return path into the `dispatch` module. After this patch, there should be no remaining places left where we call `sys.exit()` except for valid uses in the `dispatch` and `worker` modules. Differential Revision: https://phab.mercurial-scm.org/D9272