fix bug in regexp

[idnael]

There is a problem with some rules.

A gitignore pattern ".gradle"
was correctly matching ".gradle" and "app/.gradle"
but was also wrongly matching "build.gradle" and "app/build.gradle"

Other case:
A gitignore pattern "utilities/"
was wrongly matching "src/misc_utilities"

To fix it, in method fnmatch_pathname_to_regex I added a positive lookbehind
in the beginning of the regexp, that will match the string beginning or the "/" character

return r'(?:\A|(?<=/))' + ''.join(res)

I can't do a pull request.