help: add help_unknown_ref()

help: add help_unknown_ref()

When the user gives an unknown string to a command that expects to
get a ref, we could be more helpful than just saying "that's not a
ref" and die.

Add helper function help_unknown_ref() to take care of displaying an
error message along with a list of suggested refs the user might
have meant.  An interaction with "git merge" might go like this:

	$ git merge foo
	merge: foo - not something we can merge

	Did you mean one of these?
	    origin/foo
	    upstream/foo

Signed-off-by: Vikrant Varma <vikrant.varma94@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>