rev-parse: simplify dotdot parsing

rev-parse: simplify dotdot parsing

The previous commit simplified the way that revision.c parses ".." and
"..." range operators. But there's roughly similar code in rev-parse.
This is less likely to trigger a segfault, as there is no library
function which we'd pass a string literal to, but it still causes the
compiler to complain about laundering away constness via strstr().

Let's give it the same treatment, copying the left-hand side of the
range operator into its own string.

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