fileset: add a lightweight file filtering language This patch was inspired by one that Jun Wu authored for the fb-experimental repo, to avoid using matcher for efficiency[1]. We want a way to specify what files will be converted to LFS at commit time. And per discussion, we also want to specify what files to skip, text diff, or merge in another config option. The current `lfs.threshold` config option could not satisfy complex needs. I'm putting it in a core package because Augie floated the idea of also using it for narrow and sparse. Yuya suggested farming out to fileset.parse(), which added support for more symbols. The only fileset element not supported here is 'negate'. (List isn't supported by filesets either.) I also changed the 'always' token to the 'all()' predicate for consistency, and introduced 'none()' to improve readability in a future tracked file based config. The extension operator was changed from '.' to '**', to match how recursive path globs are specified. Finally, I changed the path matcher from '/' to 'path:' at Yuya's suggestion, for consistency with matcher. Unfortunately, ':' is currently reserved in filesets, so this has to be quoted to be processed as a string instead of a symbol[2]. We should probably revisit that, because it's seriously ugly. But it's only used by an experimental extension, and I think using a file based config for LFS may drive some more tweaks, so I'm settling for this for now. I reserved all of the glob characters in fileset except '.' and '_' for the extension test because those are likely valid extension characters. Sample filter settings: all() # everything size(">20MB") # larger than 20MB !**.txt # except for .txt files **.zip | **.tar.gz | **.7z # some types of compressed files "path:bin" # files under "bin" in the project root [1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-December/109387.html [2] https://www.mercurial-scm.org/pipermail/mercurial-devel/2018-January/109729.html

  $ cat > echo.py <<EOF
  > #!$PYTHON
  > from __future__ import absolute_import, print_function
  > import os
  > import sys
  > try:
  >     import msvcrt
  >     msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)
  >     msvcrt.setmode(sys.stderr.fileno(), os.O_BINARY)
  > except ImportError:
  >     pass
  > 
  > for k in ('HG_FILE', 'HG_MY_ISLINK', 'HG_OTHER_ISLINK', 'HG_BASE_ISLINK'):
  >     print(k, os.environ[k])
  > EOF

Create 2 heads containing the same file, once as
a file, once as a link. Bundle was generated with:

# hg init t
# cd t
# echo a > a
# hg ci -qAm t0 -d '0 0'
# echo l > l
# hg ci -qAm t1 -d '1 0'
# hg up -C 0
# ln -s a l
# hg ci -qAm t2 -d '2 0'
# echo l2 > l2
# hg ci -qAm t3 -d '3 0'

  $ hg init t
  $ cd t
  $ hg -q pull "$TESTDIR/bundles/test-merge-symlinks.hg"
  $ hg up -C 3
  3 files updated, 0 files merged, 0 files removed, 0 files unresolved

Merge them and display *_ISLINK vars
merge heads

  $ hg merge --tool="$PYTHON ../echo.py"
  merging l
  HG_FILE l
  HG_MY_ISLINK 1
  HG_OTHER_ISLINK 0
  HG_BASE_ISLINK 0
  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)

Test working directory symlink bit calculation wrt copies,
especially on non-supporting systems.
merge working directory

  $ hg up -C 2
  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ hg copy l l2
  $ HGMERGE="$PYTHON ../echo.py" hg up 3
  merging l2
  HG_FILE l2
  HG_MY_ISLINK 1
  HG_OTHER_ISLINK 0
  HG_BASE_ISLINK 0
  0 files updated, 1 files merged, 0 files removed, 0 files unresolved

  $ cd ..