worker: allow a function to be run in multiple worker processes
If we estimate that it will be worth the cost, we run the function in
multiple processes. Otherwise, we run it in-process.
Children report progress to the parent through a pipe.
Not yet implemented on Windows.
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
adda
$ cd ..