Fix pattern with slash in range

[jherland]

When a slash appears in a range, it should NOT match a directory
separator.

Quoting https://git-scm.com/docs/gitignore#_pattern_format:

The range notation, e.g. [a-zA-Z], can be used to match one of the
characters in a range. See fnmatch(3) and the FNM_PATHNAME flag for
a more detailed description.

And quoting what fnmatch(3) has to say about FNM_PATHNAME:

match a slash in string only with a slash in pattern and not by an
asterisk (*) or a question mark (?) metacharacter, nor by a bracket
expression ([]) containing a slash.

Hence, it seems to me that a slash inside a range should be disregarded.