tests/test-hgk.t

largefiles: simplify iteration over standins

Instead of iterating over all files in the context and ignoring those
that are not standins, pass a standin-matcher to the context and
iterate over only the files matching.

Apart from making the intent clearer, this implementation will also
benefit from any future optimizations done to the manifest walking
code.

Minimal hgk check

  $ echo "[extensions]" >> $HGRCPATH
  $ echo "hgk=" >> $HGRCPATH
  $ hg init repo
  $ cd repo
  $ echo a > a
  $ hg ci -Am adda
  adding a
  $ hg debug-cat-file commit 0
  tree a0c8bcbbb45c
  parent 000000000000
  author test 0 0
  revision 0
  branch default
  phase draft
  
  adda

  $ cd ..