Extract _internal_prep_message().
Extract _internal_prep_message().

The function internal_prep_message is kind of awkward to
call, so I'm moving most of its implementation to
_internal_prep_message() for upcoming refactorings.

timabbott/zulip,eeshangarg/zulip,tommyip/zulip,jphilipsen05/zulip,brockwhittaker/zulip,jackrzhang/zulip,verma-varsha/zulip,punchagan/zulip,andersk/zulip,brainwane/zulip,vabs22/zulip,j831/zulip,Galexrt/zulip,showell/zulip,dhcrzf/zulip,synicalsyntax/zulip,vaidap/zulip,jackrzhang/zulip,hackerkid/zulip,mahim97/zulip,christi3k/zulip,jrowan/zulip,hackerkid/zulip,christi3k/zulip,zulip/zulip,dhcrzf/zulip,Galexrt/zulip,brockwhittaker/zulip,punchagan/zulip,Galexrt/zulip,timabbott/zulip,Galexrt/zulip,tommyip/zulip,jrowan/zulip,vaidap/zulip,mahim97/zulip,jackrzhang/zulip,shubhamdhama/zulip,verma-varsha/zulip,j831/zulip,showell/zulip,punchagan/zulip,jphilipsen05/zulip,eeshangarg/zulip,jrowan/zulip,amanharitsh123/zulip,vaidap/zulip,amanharitsh123/zulip,jackrzhang/zulip,ryanbackman/zulip,timabbott/zulip,showell/zulip,hackerkid/zulip,andersk/zulip,shubhamdhama/zulip,kou/zulip,christi3k/zulip,amanharitsh123/zulip,brainwane/zulip,amanharitsh123/zulip,zulip/zulip,j831/zulip,Galexrt/zulip,kou/zulip,brainwane/zulip,shubhamdhama/zulip,dhcrzf/zulip,ryanbackman/zulip,showell/zulip,eeshangarg/zulip,zulip/zulip,ryanbackman/zulip,rht/zulip,timabbott/zulip,j831/zulip,eeshangarg/zulip,mahim97/zulip,brockwhittaker/zulip,shubhamdhama/zulip,synicalsyntax/zulip,rht/zulip,christi3k/zulip,jackrzhang/zulip,ryanbackman/zulip,synicalsyntax/zulip,showell/zulip,brockwhittaker/zulip,mahim97/zulip,rishig/zulip,rht/zulip,Galexrt/zulip,hackerkid/zulip,shubhamdhama/zulip,verma-varsha/zulip,andersk/zulip,tommyip/zulip,kou/zulip,hackerkid/zulip,vabs22/zulip,ryanbackman/zulip,vaidap/zulip,synicalsyntax/zulip,christi3k/zulip,jphilipsen05/zulip,shubhamdhama/zulip,vabs22/zulip,eeshangarg/zulip,tommyip/zulip,verma-varsha/zulip,vabs22/zulip,rht/zulip,mahim97/zulip,dhcrzf/zulip,vaidap/zulip,synicalsyntax/zulip,zulip/zulip,tommyip/zulip,jackrzhang/zulip,christi3k/zulip,j831/zulip,punchagan/zulip,rishig/zulip,dhcrzf/zulip,kou/zulip,punchagan/zulip,rishig/zulip,ryanbackman/zulip,synicalsyntax/zulip,Galexrt/zulip,brainwane/zulip,verma-varsha/zulip,eeshangarg/zulip,jrowan/zulip,amanharitsh123/zulip,mahim97/zulip,andersk/zulip,tommyip/zulip,zulip/zulip,zulip/zulip,showell/zulip,brockwhittaker/zulip,hackerkid/zulip,kou/zulip,brainwane/zulip,kou/zulip,showell/zulip,jackrzhang/zulip,hackerkid/zulip,vaidap/zulip,brockwhittaker/zulip,rishig/zulip,kou/zulip,jphilipsen05/zulip,brainwane/zulip,andersk/zulip,timabbott/zulip,rht/zulip,jrowan/zulip,rishig/zulip,eeshangarg/zulip,dhcrzf/zulip,jphilipsen05/zulip,rht/zulip,punchagan/zulip,andersk/zulip,brainwane/zulip,synicalsyntax/zulip,amanharitsh123/zulip,punchagan/zulip,timabbott/zulip,rishig/zulip,rishig/zulip,dhcrzf/zulip,vabs22/zulip,jrowan/zulip,vabs22/zulip,rht/zulip,shubhamdhama/zulip,verma-varsha/zulip,timabbott/zulip,jphilipsen05/zulip,andersk/zulip,j831/zulip,tommyip/zulip,zulip/zulip