Fix bug popping ignore files during wd iteration

There were a couple bugs in popping ignore files during iteration
that could result in incorrect decisions be made and thus ignore
files below the root either not being loaded correctly or not
being popped at the right time.

One bug was an off-by-one in comparing the path of the gitignore
file with the path being exited during iteration.

The second bug was not correctly truncating the path being tracked
during traversal if there were no ignores on the list (i.e. when
you have no .gitignore at the root, but do have some in contained
directories).
Merge pull request #2262 from libgit2/rb/fix-ignore-pop

Fix bug popping ignore files during wd iteration