Correctly handle common prefixes in substring completion

Correctly handle common prefixes in substring completion

Substring completion would previously not complete the longest common
substring if that substring was a prefix of all the completion
alternatives.  Now it does.  An explanation of this bug

Substring completion is implemented by passing the `prefix' symbol as
part of the pattern passed to completion-pcm--merge-completions.  This
symbol is supposed to cause completion-pcm--merge-completions to
"grow" a completion of a common substring only from the "right" of the
symbol (a common suffix), not from the "left" of the symbol (a common
prefix).  Yes, this is the opposite of what the name `prefix' would
imply.

When processing a symbolic element of the pattern,
completion-pcm--merge-completions first finds the common prefix of all
the completions in that part of the pattern (using try-completion).
Then for `prefix' and other elements which want to complete a common
suffix, the common prefix is removed from each element and then the
common suffix is calculated with completion--common-suffix.

If the common prefix covers the entirety of all the alternatives
(i.e. when "unique" is true in the code), it's also a common suffix.
In that case, the common suffix calculation (if it runs) is basically
a no-op which will produce an empty string, since we removed the
common prefix before running it.

Before this change, `prefix' elements would unconditionally discard
the common prefix, which produced the wrong result in the case that
common prefix == common suffix.  For example:

  (completion-pcm--merge-completions '("ab" "ab") '(prefix "b"))
  -> ("b")

Now we detect this situation and include the common prefix in this
case for `prefix' elements.  Then we get:

  (completion-pcm--merge-completions '("ab" "ab") '(prefix "b"))
  -> ("b" "a")

which is correct.

* lisp/minibuffer.el (completion-pcm--merge-completions): Don't ignore
a common suffix in a `prefix' pattern element when it's also a common
prefix.
* test/lisp/minibuffer-tests.el (completion-substring-test-5): Add a
test.