Add directory pattern matching to attributes

The manpage of gitattributes says: "The rules how the pattern
matches paths are the same as in .gitignore files" and the gitignore
pattern matching has a pattern ending with / for directory matching.

This rule is specifically relevant for the 'export-ignore' rule used
for git archive.

Signed-off-by: Jean-Noel Avila <jn.avila@free.fr>