run-tests.py: add RUNTESTDIR to refer `tests` of Mercurial Before this patch, there is no way to refer files under `tests` or so of Mercurial source tree, when `run-tests.py` runs in `tests` of third party tools. In this case, `TESTDIR` refers the latter `tests`. This prevents third party tools from using useful tools in Mercurial source tree (e.g. `contrib/check-code.py`). This patch adds `RUNTESTDIR` environment variable to refer `tests` of Mercurial source tree, in which `run-tests.py` now running is placed. For example, tests of third party tools can refer `contrib/check-code.py` in Mercurial source tree as `$RUNTESTDIR/../contrib/check-code.py`. BTW, for similarity with `TESTDIR` referring `test*s*` directory, newly added environment variable isn't named as `RUNTEST*S*DIR`. In addition to it, the corresponded local variable is also named as `runtestdir`. This is a one of preparations for issue4677.

run-tests.py: execute hghave by the path relative to run-tests.py Before this patch, `run-tests.py` executes `hghave` by the path relative to `TESTDIR` (= cwd of `run-tests.py` running). This prevents third party tools for Mercurial from running `run-tests.py`, which is placed in `tests` of Mercurial source tree, in `tests` of own source tree. In such cases, `TESTDIR` refers the latter `tests`, and `hghave` doesn't exist in it. This is a one of preparations for issue4677.

templatekw: make {latesttag} a hybrid list This maintains the previous behavior of expanding {latesttag} to a string containing all of the tags, joined by ':'. But now it also allows list type operations. I'm unsure if the plural handling is correct (i.e. it seems like it is usually "{foos % '{foo}'}"), but I guess we are stuck with this because the singular form previously existed.

templatekw: allow the caller of showlist() to specify the join() separator The keyword {latesttag} currently manually joins the list of tags using ':', which prevents a transparent switch over to a hybrid list.

archive: use {changessincelatesttag} to build the metadata file This isolates the current magic when dealing with wdir() and only().

templatekw: introduce the changessincelatesttag keyword Archive is putting a value with the same name in the metadata file, to count all of the changes not covered by the latest tag, instead of just along the longest path. It seems that this would be useful to have on the command line as well. It might be nice for the name to start with 'latesttag' so that it is grouped with the other tag keywords, but I can't think of a better name. The initial version of this counted a clean wdir() and '.' as the same value, and a dirty wdir() as the same value after it is committed. Yuya objected on the grounds of consistency [1]. Since revsets can be used to conditionally select a dirty wdir() or '.' when clean, I can build the version string I need and will defer to him on this. [1] https://www.selenic.com/pipermail/mercurial-devel/2015-June/071588.html

help: support 'hg help template.somekeyword' Previously the output was simply 'abort: help section not found'.