util._hgexecutable: default to 'hg' This should make hooks work even if util.set_hgexecutable is not called (e.g. hgweb.cgi). Fixes issue625.

abort: Failed to parse .../t/.hg/hgrc
File contains no section headers.
file: .../t/.hg/hgrc, line: 1
'invalid\n'