gitweb: avoid stale variable contents

gitweb: avoid stale variable contents

When gitweb is running in an optimized CGI processing mode
(mod_perl, FCGI, PSGI, etc.), multiple requests are served
by the same invocation of gitweb.cgi.

It is crucial for proper operation that remnants from the
previous request are not allowed to taint subsequent requests.

In particular, there are a number of "our" variables that
need to be expliictly cleared in order to prevent this from
happening and possibly corrupting subsequent requests.

The snapshot action is particularly susceptible to corruption
without these precautions.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>