Fix issue 1766 - bugs in managing ignore file lists

The routines to push and pop ignore files while traversing a directory hierarchy had some issues. In particular, setting up the initial list would sometimes push an ignore file before it ought to be applied if the starting path was a directory containing an ignore file. Also, the pop function was not always matching the right part of the path and would fail to pop ignores from the list in some cases.

This adds some tests that exercise a particular problematic case and then fixes the problems that I could find related to this.

At some point, I'd like to isolate this ignore rule management code and rewrite it, but that's a larger project and right now, I'll opt to just try to fix the broken behaviors.

Fixes #1766