gitweb: optimize $lastactivity_file code

gitweb: optimize $lastactivity_file code

Don't waste time checking to see if the file is readable before
trying to open it for reading, just check the result of an open-
for-reading call.

Also ignore a failure of close since we validate the line we
read the close exit status is irrelevant.

These changes also have the side-effect of eliminating use
of 'last' that would have incorrectly exited the function
if it ever triggered.

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