compat: fix fixedcopytracing compatibility with mercurial 4.8 and narrow
Mercurial 4.8 version of _fullcopytracing includes some new narrow code. As
https://phab.mercurial-scm.org/D3896 has not yet landed, update
fixedcopytracing behind a version detection condition. This was spotted by
Augie Fackler.
#require test-repo pyflakes
Copied from Mercurial core (60ee2593a270)
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null