Fixed support for ignore file regexes that do not contain a slash.

This small patch contains a fix that addresses the problem of correctly dealing with regexes that do not contain the '/' character (such as "*~", "*.swp", etc.) in (.ag/.git)ignore files.

Basically, the rationale of this patch is not to enable the FNM_PATHNAME flag for fnmatch unless the regex actually contains a '/' character.

I tested the patch on OS X 10.9, and it seems to work fine.