ui: add prompt argument to write (issue5154) (API)

When code like filemerge._iprompt calls ui.prompt, it expects
the user to see the output in addition to getting the prompt.

Other code such as histedit may call ui.pushbuffer, but its
goal is not to interfere with prompts, so this commit adds
an optional prompt flag to ui.write and has _readline
include that argument.

ui.promptchoice calls ui.prompt which calls ui._readline.

This commit also updates hgext.color.write.

hgext/color.py

@@ -424,7 +424,7 b' class colorui(uimod.ui):'
             return super(colorui, self).write(*args, **opts)
 
         label = opts.get('label', '')
-        if self._buffers:
+        if self._buffers and not opts.get('prompt', False):
             if self._bufferapplylabels:
                 self._buffers[-1].extend(self.label(a, label) for a in args)
             else:

mercurial/ui.py

@@ -662,7 +662,7 b' class ui(object):'
         "cmdname.type" is recommended. For example, status issues
         a label of "status.modified" for modified files.
         '''
-        if self._buffers:
+        if self._buffers and not opts.get('prompt', False):
             self._buffers[-1].extend(a for a in args)
         else:
             self._progclear()
@@ -842,7 +842,7 b' class ui(object):'
 
         # call write() so output goes through subclassed implementation
         # e.g. color extension on Windows
-        self.write(prompt)
+        self.write(prompt, prompt=True)
 
         # instead of trying to emulate raw_input, swap (self.fin,
         # self.fout) with (sys.stdin, sys.stdout)