util._hgexecutable: default to 'hg'
This should make hooks work even if util.set_hgexecutable is not called
(e.g. hgweb.cgi).
Fixes issue625.
% simple
port:4:export
port:4:vaportight
port:4:import/export
% all
port:4:4:-:spam:import/export
port:3:4:+:eggs:import/export
port:2:1:-:spam:import
port:2:2:-:spam:export
port:2:1:+:spam:export
port:2:2:+:spam:vaportight
port:2:3:+:spam:import/export
port:1:2:+:eggs:export
port:0:1:+:spam:import
% other
port:4:import/export
% follow
port:0:import
port2:6:4:+:eggs:deport
port:4:4:-:spam:import/export
port:3:4:+:eggs:import/export
port:2:1:-:spam:import
port:2:2:-:spam:export
port:2:1:+:spam:export
port:2:2:+:spam:vaportight
port:2:3:+:spam:import/export
port:1:2:+:eggs:export
port:0:1:+:spam:import
color:3:orange
color:3:+:orange
color:2:-:orange
color:1:+:orange