gitweb: fix wrong base URL when non-root DirectoryIndex

gitweb: fix wrong base URL when non-root DirectoryIndex

CGI::url() has some issues when rebuilding the script URL if the script
is a DirectoryIndex.

One of these issue is the inability to strip PATH_INFO, which is why
we had to do it ourselves.

Another issue is that the resulting URL cannot be used for the <base>
tag: it works if we're the DirectoryIndex at the root level, but not
otherwise.

We fix this by building the proper base URL ourselves, and improve the
comment about the need to strip PATH_INFO manually while we're at it.

Additionally t/t9500-gitweb-standalone-no-errors.sh had to be modified
to set SCRIPT_NAME variable (CGI standard states that it MUST be set,
and now gitweb uses it if PATH_INFO is not empty, as is the case for
some of tests in t9500).

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