discovery: prefer loop to double-for list comprehension in changegroupsubset
The double-for form of list comprehensions gets particularly unreadable
when you throw in an 'if' condition. This expands the only remaining
instance of the double-for syntax in our codebase into a loop.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a