Don't resort Icomplete candidates when default already on top

Don't resort Icomplete candidates when default already on top

Fixes: bug#43222
Icomplete mode re-sorts candidates, bubbling the default to top if
it's found somewhere down the list.  This is done according to two
criteria: exact match and prefix match.  Before this fix, it didn't
take into account the possibility that the exact match for the default
would already be on top, and would incorrectly bubble a prefixing
completion down the list to the top.  This commit fixes that.

* lisp/icomplete.el (icomplete--sorted-completions):
Rework. Recomment.