Change bad_ref_char() to return a boolean value

Change bad_ref_char() to return a boolean value

Previously most bad characters were indicated by returning 1, but "*"
was special-cased to return 2 instead of 1.  One caller examined the
return value to see whether the special case occurred.

But it is easier (to document and understand) for bad_ref_char()
simply to return a boolean value, treating "*" like any other bad
character.  Special-case the handling of "*" (which only occurs in
very specific circumstances) at the caller.  The resulting calling
code thereby also becomes more transparent.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>