ui: flush before prompting for input with readline
I was using `hg absorb` with Python 3 and noticed that the
prompt was appearing without any output about what would be
done. After I answered the prompt, the output was printed
to stdout.
This appears to be a buffering difference between Python 2
and Python 3.
To work around it, this commit adds an explicit flush() before
calling the raw input function when readline is used.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}