engine: prevent a function call for each store file
Python function calls are not cheap. Instead of calling the function once for
each file in store, we use a dedicated function which filters and yields the
required files.
Differential Revision: https://phab.mercurial-scm.org/D9673
#require black test-repo
$ cd $RUNTESTDIR/..
$ black --config=black.toml --check --diff `hg files 'set:(**.py + grep("^#!.*python")) - mercurial/thirdparty/**'`