use "\&" where possible when referring to subroutines

use "\&" where possible when referring to subroutines

"*foo" is ambiguous in that it may refer to a bareword file handle;
so we'll use it where we can without triggering warnings.

PublicInbox::TestCommon::run_script_exit required dropping the
prototype, however.  We'll also future-proof by dropping "use
warnings" in Cgit.pm and use the less-ambiguous "//=" in Inbox.pm
while we're in the area.