gitweb: fix project list if PATH_INFO=="/".

gitweb: fix project list if PATH_INFO=="/".

The project list now uses several common header / footer generation functions.
These functions only check for "defined $project", but when PATH_INFO just
contains a "/" (which is often generated by web servers), then this test
fails.

Now explicitly undef $project if there is none so that the tests in other
gitweb parts work again.

Signed-off-by: Martin Waitz <redacted>
Signed-off-by: Junio C Hamano <redacted>