When using either of
set-option g completers option=my_option
prompt -shell-script-candidates ...
While the search text is empty, the completions will be sorted
alphabetically.
This is bad because it means the most important entries are not listed
first, making them harder to select or even spot.
Let's apply input order before resorting to sorting alphabetically.
In theory there is a more elegant solution: sort candidates (except
if they're user input) before passing them to RankedMatch, and then
always use stable sort. However that doesn't work because we use a
heap which doesn't support stable sort.
Closes#1709, #4813
658c3385a9