util._hgexecutable: default to 'hg'
This should make hooks work even if util.set_hgexecutable is not called
(e.g. hgweb.cgi).
Fixes issue625.
error: pretxncommit.nocommits hook failed: no commits allowed
abort: no commits allowed
transaction abort!
rollback completed
error: pretxncommit.nocommits hook failed: no commits allowed
abort: no commits allowed
transaction abort!
rollback completed