In order to match the star-star, we disable the flag that's looking for
a single path element, but that leads to searching for the pattern in
the middle of elements in the input string.

Mark when we're handing a star-star so we jump over the elements in our
attempt to match the part of the pattern that comes after the star-star.

While here, tighten up the check so we don't allow invalid rules
through.

If we're looking for a symlink, realpath will give us the resolved path,
which is not what we're after, but a canonicalized version of the path
the user asked for.

When a .gitignore specifies some folder "foo/", ensure that a file
with the same name "foo" is not ignored.

Ensure that when examining a .gitignore in a subdirectory, we do not
erroneously apply the paths contained therein to the root of the
repository.  (Fixed in c02a0e4).

There are a few tests that set up a fake home directory and a
fake GLOBAL search path so that we can test things in global
ignore or attribute or config files.  This cleans up that code to
work more robustly even if there is a test failure.  This also
fixes some valgrind warnings where scanning search paths for
separators could end up doing a little bit of sketchy data access
when coming to the end of search list.

This allows "foo/**/*.html" to match "foo/file.html"