merge: move the filtering of ambiguous files to a dedicated function
I have multiple reasons:
- The body of `_update` is way too long
- This adds typing which will help our tooling and brains understand this
code more easily
- This function will get more nested and complex in the next patch
I've taken the liberty of rewrapping and typo-passing the docstring.
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present