Enable default value for head parameter

Pages which expect head to be specified in the querystring can now be
given a default value, configurable per repository (via repo.defbranch,
which defaults to "master").

Currently, only the log page actually works without parameters, but the
defbranch is bound to be exploited.

This also removes some dead code from shared.c

Signed-off-by: Lars Hjemli <hjemli@gmail.com>