Demonstrate a trailing slash failure.

When ignoring a parent folder and negating two child folders with and without trailing slashes respectively, libgit2 is ignoring the one without but not the one with:

    parent/*
    parent/child1
    parent/child2/

`git help ignore` has this to say about trailing slashes:

> If the pattern ends with a slash, it is removed for the purpose of the following description, but it would only find a match with a directory. In other words, foo/ will match a directory foo and paths underneath it, but will not match a regular file or a symbolic link foo (this is consistent with the way how pathspec works in general in Git).

Sure enough, having manually performed the same steps as this test, `git status` tells us the following:

	# On branch master
	#
	# Initial commit
	#
	# Changes to be committed:
	#   (use "git rm --cached <file>..." to unstage)
	#
	#	new file:   force.txt
	#
	# Untracked files:
	#   (use "git add <file>..." to include in what will be committed)
	#
	#	../.gitignore
	#	child1/
	#	child2/

i.e. neither child1 nor child2 is ignored.

This test is closely based on #2406, written by @carlosmn.