destination: honor the sourceset argument of _destmerge
Mercurial gained a "sourceset" argument to compute the merge destination of any
changeset (the one in the 'sourceset'). This now works for the version
in topic too. The support is quite basic (because we pick only one changeset
from the sourceset) but that's a start.
#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")' 2>/dev/null \
> | xargs pyflakes 2>/dev/null