bitbake/fetch: Fix uri_replace

bitbake/fetch: Fix uri_replace

We were seeing duplicate endings to urls:

DEBUG: For url ['http', 'www.apache.org', '/dist/subversion/subversion-1.7.1.tar.bz2', '', '', {}] comparing ['http', 'www.apache.org', '/dist', '', '', {}] to ['http', 'archive.apache.org', '/dist', '', '', {}]
DEBUG: For url http://www.apache.org/dist/subversion/subversion-1.7.1.tar.bz2 returning http://archive.apache.org/dist/subversion/subversion-1.7.1.tar.bz2/subversion-1.7.1.tar.bz2

This patch addresses this by only performing substitutions when really needed.

(Bitbake rev: a17473d51f54a9dfb4eeaa5ad516c4851d5ac142)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>