bug#65137: 29.1; completion-substring-try-completion doesn't return the longest common substring

[Spencer Baugh]

I've tracked it down further to:

(completion-pcm--merge-completions
 '("ab1" "ab2")
 '(prefix "b"))
-> (any "b")

which definitely doesn't match the docstring of
completion-pcm--merge-completions...  (note that it returns elements in
reverse order, so this is basically just returning the string "b")