util._hgexecutable: default to 'hg'
This should make hooks work even if util.set_hgexecutable is not called
(e.g. hgweb.cgi).
Fixes issue625.
#header#<title>Mercurial Error</title></head><body><h2>Mercurial Error</h2><p>An error occured while processing your request:</p><p>#error|escape#</p>#footer#