When a glob pattern ended with a \/, and since we permit backslash
escapes, the glob parser gave a "dangling escape" error. Which is weird,
because the \ is clearly not dangling.
The issue is that the layer above the glob parser, the gitignore parser,
was stripping the trailing / so that it wouldn't be part of the matching
logic. Of course, stripping the trailing / while it is escaped without
removing the backslash escape is wrong. So we do that here.
Fixes#2236
9f0e88bcb1