gitweb: make static files accessible with PATH_INFO

gitweb: make static files accessible with PATH_INFO

Gitweb links to a number of static files such as CSS stylesheets,
favicon or the git logo. When, such as with the default Makefile, the
paths to these files are relative (i.e. doesn't start with a "/"), the
files become inaccessible in any view other tha project list and summary
page if gitweb is invoked with a non-empty PATH_INFO.

Fix this by adding a <base> element pointing to the script's own URL,
which ensure that all relative paths will be resolved correctly.

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