util._hgexecutable: default to 'hg'
This should make hooks work even if util.set_hgexecutable is not called
(e.g. hgweb.cgi).
Fixes issue625.
adding ab renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3a renamed from b:9a263dd772e0159ddfa70fb8a448bb6c7ec8c69f% test with --revb renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3