Filter is checking for '%' incorrectly.

The filter code for checking for a '%' in src/libgit2/filter-filter.c uses the following code which is incorrect:

ssize_t foundPercentage = git_buf_find(buf, '%');
if (foundPercentage) {

The git_buf_find() function returns a -1 if it does not find the character and not a 0 as the existing code assumes.