transaction: write pending generated files
Such file are generated with a .pending prefix. It is up to the reader to
implement the necessary logic for reading pending files.
We add a test to ensure pending files are properly cleaned-up in both success and
error cases.
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 ..