Preserve an explicit * in pcm-try-completion

Preserve an explicit * in pcm-try-completion

An explicitly typed * has different semantics from automatically
inserted PCM wildcards, so it should be preserved on try-completion.  We
already do this in some cases, but now we do it more.

This is especially significant for filename completion: removing an
explicit * can take us from

~/src/emacs/trunk/*/minibuf

to

~/src/emacs/trunk//minibuf

The explicit double slash is interpreted by the file name completion
table to mean "start completing from the root directory", so deleting
the * here substantially changes semantics.

* lisp/minibuffer.el (completion-pcm--merge-completions): Don't drop
important wildcards. (bug#74420)
* test/lisp/minibuffer-tests.el (completion-pcm-test-7): Add tests.

(cherry picked from commit 0fbba16387513e7692b46885833e4a9c218251f0)