extdata: add extdatasource reader This adds basic support for extdata, a way to add external data sources for revsets and templates. An extdata data source is simply a list of lines of the form: <revision identifier>[<space><freeform text>]\n An extdata source is configured thusly: [extdata] name = <a url or path> urls of the form shell: are launch shell commands to generate data. This patch is slightly modified by Yuya Nishihara as follows: - fix typo - remove unused function - remove future expansion point for parameter (which can be added later as the extdata revset/template are experimental) You can see the original patch at https://www.mercurial-scm.org/pipermail/mercurial-devel/2016-September/088426.html

tests: change to parent directory before all "hg init" in test-log.t Add a "cd .." before the second "hg init" so that all repositories are on top level of test directory. Makes inspection of test directory easier.

config: add a missing preparewrite() call Thanks Yuya for pointing this out in D808. Differential Revision: https://phab.mercurial-scm.org/D924

test-strip: make test compatible with chg The test was using reposetup which had the logic for stripping commits. This leads to a situation where if the reposetup was called twice for an extension (which can happen with chg running), the stripped node would not be found the second time. Therefore, this commit changes the test to put the stripping logic inside commands instead of the reposetup. This ensures that the stripping logic is invoked only when the command is invoked and thus, avoids any problems. Test Plan: Ran the test 'test-strip.t' with and without the '--chg' option. Differential Revision: https://phab.mercurial-scm.org/D928