ui-plain.c: only return the blob with the specified path

When a path to a directory was specified for the 'plain'
view, each blob in the directory used to be returned to
the client. This patch fixes the issue by matching the
path of each blob against the requested path.

Noticed-by: Lars Stoltenow <penma@penma.de>
Signed-off-by: Lars Hjemli <larsh@slackbox.hjemli.net>