gitweb/README: fix AliasMatch in example

gitweb/README: fix AliasMatch in example

When combining "dumb client" and human-friendly access by using the
'.git' extension to switch between the two, make sure the AliasMatch
covers the entire request. Without a full match, a request for

http://git.example.com/project/shortlog/branch..gitsomething

would result in a 404 because the server would try to access the
the project 'project/shortlog/branch.'

The solution is still not bulletproof, so document the possible failing
case.

Signed-off-by: Giuseppe Bilotta <redacted>
Signed-off-by: Junio C Hamano <redacted>