pathspec: factor out magic-to-name function

pathspec: factor out magic-to-name function

When we have unsupported magic in a pathspec (because a command or code
path does not support particular items), we list the unsupported ones in
an error message.

Let's factor out the code here that converts the bits back into their
human-readable names, so that it can be used from other callers, which
may want to provide more flexible error messages.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>