ui-plain: fix to show a repo's root directory listing in plain view

This is to fix the case of accessing http://host.com/cgit.cgi/repo.git/plain/

There is code here to make this case work (match_baselen is set to -1
for top-of-the-tree views) but the unsigned to signed comparison was
always false in this case, causing an empty directory listing without
this fix.

Signed-off-by: Joe Anakata <jea-signup-github@anakata.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>