hgweb: fix handling of arguments in the between command
The 'pairs' argument was coded to be optional, but the code would
crash if it was not provided.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://mercurial.selenic.com/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.