upload-pack: fix ambiguous error message

upload-pack: fix ambiguous error message

upload-pack.c takes any --shallow-exclude argument(s) from
clone/fetch/etc. and passes them through expand_ref().  If it does not
get back exactly one ref from the call to expand_ref(), it will die with
the following error:

    fatal: git upload-pack: ambiguous deepen-not: %s

Given that the documentation suggests to users that --shallow-exclude
accepts a revision rather than a ref (which will be corrected in a
subsequent commit), users may try to pass a revision.  In such a case,
expand_ref() will return 0 matches, but the error message we print will
be misleading since "ambiguous" suggests there are multiple matches.
Provide a clearer error message for such a case.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>