narrow: import experimental extension from narrowhg revision cb51d673e9c5 Adjustments: * renamed src to hgext/narrow * marked extension experimental * added correct copyright header where it was missing * updated hgrc extension enable line in library.sh * renamed library.sh to narrow-library.sh * dropped all files from repo root as they're not interesting * dropped test-pyflakes.t, test-check-code.t and test-check-py3-compat.t * renamed remaining tests to all be test-narrow-* when they didn't already * fixed test-narrow-expanddirstate.t to refer to narrow and not narrowhg * fixed tests that wanted `update -C .` instead of `merge --abort` * corrected a two-space indent in narrowspec.py * added a missing _() in narrowcommands.py * fixed imports to pass the import checker * narrow only adds its --include and --exclude to clone if sparse isn't enabled to avoid breaking test-duplicateoptions.py. This is a kludge, and we'll need to come up with a better solution in the future. These were more or less the minimum to import something that would pass tests and not create a bunch of files we'll never use. Changes I intend to make as followups: * rework the test-narrow-*-tree.t tests to use the new testcases functionality in run-tests.py * remove lots of monkeypatches of core things Differential Revision: https://phab.mercurial-scm.org/D1974