util._hgexecutable: default to 'hg'
This should make hooks work even if util.set_hgexecutable is not called
(e.g. hgweb.cgi).
Fixes issue625.
#!/bin/shhg initecho a > ahg ci -Am thg mv a bhg ci -Am t1hg debugrename bhg mv b ahg ci -Am t2hg debugrename aecho % test with --revhg debugrename --rev 1 b