bug#28439: suggestion: support case-independent xref-find-definitions prompt

[Winston]

[I've corrected the Subject and Cc lists above to refer to bug#28439
 instead of sending to bug-gnu-emacs@gnu.org.]

>> Sorry, what don't we want to follow the value of completion-ignore-case?

> Because it has a much broader effect.  And because the issue here is
> inconsistency between completion and actual matching in
> xref-find-definitions.  They should be consistent, IMO.

   Thank you, Eli.  IMHO, too.

   For now, I'm using completion-ignore-case because it solves the
immediate issue, but I was concerned that it could have side-effects
elsewhere that I won't discover until they happen.

   Sure, since I like it here, I might like it elsewhere, but I won't
know until those situations come up, and if I don't like it elsewhere,
then I'd be stuck.

   I liked your patch when I saw it, because it directly addressed the
issue.  Other options obviously include:
* xref-completion-ignore-case, default t;
* creating something that applies to both completions and find-definition
  (I didn't save the patch, but my vague recollection is that it just
  defaulted to using t for completions).

JMO, FWIW.
 -WBE