ui: add explicit path to write prompt text bypassing buffers
The prompt= flag was added at e35d7f131483, when colorui had its own write()
function. Since we've merged colorui to ui, we can simply call the unbuffered
write() function.
#require docutils gettext
$ $TESTDIR/check-gendoc de
checking for parse errors
Die Dateien werden dem Projektarchiv beim n\xc3\xa4chsten \xc3\x9cbernehmen (commit) hinzugef\xc3\xbcgt. Um dies vorher r\xc3\xbcckg\xc3\xa4ngig zu machen, siehe:hg:`forget`. (esc)
warning: please have a space before :hg: