ui.prompt was completely silent in non-interactive mode, unless in verbose
mode. It is fine that it chooses the default automatically, but it is confusing
that the message and prompt shown interactively can't be found in scripted
tests.
The prompt and selection is now .write'ed instead of .note'ed.
It seems like the old behaviour with different handling for commands with and
without path was intended, but I think this behaviour of util.find_exe is
better:
* Always returns existing file
* or None if command not found - no default
* Windows: Returned file thus always ends with extension from PATHEXT
This fixes http://www.selenic.com/mercurial/bts/issue1459. The change might
fix other unintended behaviour too.
