util._hgexecutable: default to 'hg'
This should make hooks work even if util.set_hgexecutable is not called
(e.g. hgweb.cgi).
Fixes issue625.
*** failed to import extension badext: bit bucket overflowextension 'hgext.gpg' overrides commands: sigs sigcheck signhg help [COMMAND]show help for a command, extension, or list of commands