the_silver_searcher: Doesn't respect "match-beginning" gitignore patterns in subdirs

In .gitignore files, ignore patterns can be prefixed with a “/”. man gitignore says:

A leading slash matches the beginning of the pathname. For example, "/*.c" matches "cat-file.c"
but not "mozilla-sha1/sha1.c".

ag currently doesn’t apply such patterns when they match files in subdirectories. Steps to reproduce:

% mkdir ag-test                                                                                            % cd ag-test                                                                                               % mkdir ignored
% echo 'hello' > ignored/a
% echo 'hello' > b
% ag hello
b
1:hello

ignored/a
1:hello
% echo '/ignored/*' > .gitignore  ## should ignore "a", but doesn't
% ag hello
b
1:hello

ignored/a
1:hello
% echo 'ignored/*' > .gitignore
% ag hello
b
1:hello

However, the non-subdir case currently works correctly:

% echo '/b' > .gitignore
% ag hello
ignored/a
1:hello

Other than this minor hiccup, it’s awesomeness all the way. 😃