sparse: use vfs.tryread() vfs.exists() followed by a file read is an anti-pattern because it incurs an extra stat() to test for file presence. vfs.tryread() returns empty string on missing file and avoids the stat().

sparse: refactor sparsechecksum() This was relying on garbage collection to close the opened file, which is a bug. Both callers simply called into self.vfs to resolve the path. So refactor to use the vfs layer. While we're here, rename the method to reflect it is internal and to break anyone relying on the old behavior.

sparse: document config file format This was previously undocumented. Seems useful to have.

sparse: rename command to debugsparse Sparse checkout is still highly experimental and not protected by BC guarantees yet. We also haven't had a discussion on the UX. To discourage use, we rename the sparse command to debugsparse.

sparse: remove reference to simplecache This is a 3rd party extension authored by Facebook. References in core are not appropriate. It will be possible to restore this code/optimization via monkeypatching. So Facebook won't lose any functionality. The removed code is important for performance. So add a comment tracking it.

sparse: remove reference to hgwatchman This is a legacy extension. Now that the extension is in core, we only need to support what's in core, which is fsmonitor.

sparse: expand module docstring Clarify lack of BC guarantees. And say a bit more about the extension.

sparse: vendor Facebook-developed extension Facebook has developed an extension to enable "sparse" checkouts - a working directory with a subset of files. This feature is a critical component in enabling repositories to scale to infinite number of files while retaining reasonable performance. It's worth noting that sparse checkout is only one possible solution to this problem: another is virtual filesystems that realize files on first access. But given that virtual filesystems may not be accessible to all users, sparse checkout is necessary as a fallback. Per mailing list discussion at https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-March/095868.html we want to add sparse checkout to the Mercurial distribution via roughly the following mechanism: 1. Vendor extension as-is with minimal modifications (this patch) 2. Refactor extension so it is more clearly experimental and inline with Mercurial practices 3. Move code from extension into core where possible 4. Drop experimental labeling and/or move feature into core after sign-off from narrow clone feature owners This commit essentially copies the sparse extension and tests from revision 71e0a2aeca92a4078fe1b8c76e32c88ff1929737 of the https://bitbucket.org/facebook/hg-experimental repository. A list of modifications made as part of vendoring is as follows: * "EXPERIMENTAL" added to module docstring * Imports were changed to match Mercurial style conventions * "testedwith" value was updated to core Mercurial special value and comment boilerplate was inserted * A "clone_sparse" function was renamed to "clonesparse" to appease the style checker * Paths to the sparse extension in tests reflect built-in location * test-sparse-extensions.t was renamed to test-sparse-fsmonitor.t and references to "simplecache" were removed. The test always skips because it isn't trivial to run it given the way we currently run fsmonitor tests * A double empty line was removed from test-sparse-profiles.t There are aspects of the added code that are obviously not ideal. The goal is to make a minimal number of modifications as part of the vendoring to make it easier to track changes from the original implementation. Refactoring will occur in subsequent patches.

contrib: widen "direct use of `python`" net again I think I've now caught all of them. Differential Revision: https://phab.mercurial-scm.org/D15

tests: clean up a newly-introduced instance of `python` Differential Revision: https://phab.mercurial-scm.org/D16

tests: clean up even more direct `python` calls with $PYTHON This time ones that are prefixed with =, ", ', or `. This appears to be the last of them. Differential Revision: https://phab.mercurial-scm.org/D14

contrib: widen the "don't use `python`" net a little I'm still cleaning this up, but it's easier to do in bite-size chunks like this than all at once. The negative lookahead avoids one false positive category from some output related to finding Subversion bindings. Differential Revision: https://phab.mercurial-scm.org/D13

followlines: join merge parents line ranges in blockdescendants() (issue5595) In blockdescendants(), we had an assertion when line range of a merge changeset was not consistent depending on which parent was considered for computation. For instance, this might occur when file content (in lookup range) is significantly different between parent branches of the merge as demonstrated in added tests (where we almost completely rewrite the "baz" file while also introducing similarities with its content in the other branch we later merge to). Now, in such case, we combine line ranges from all parents by storing the envelope of both line ranges. This is conservative (the line range is extended, possibly unnecessarily) but at least this should avoid missing descendants with changes in a range that would fall in that of one parent but not in another one (the case of "baz: narrow change (2->2+)" changeset in tests).

workingfilectx: add exists, lexists Switch the lone call in merge.py to use it. As with past refactors, the goal is to make wctx hot-swappable with an in-memory context in the future. This change should be a no-op today.

vfs: add explanation about cost of checkambig=True in corner case