diff-no-index: correctly diagnose error return from diff_opt_parse()

diff-no-index: correctly diagnose error return from diff_opt_parse()

diff_opt_parse() returns the number of options parsed, or often
returns error() which is defined to return -1.  Yes, return value of
0 is "I did not process that option at all", which should cause the
caller to say that, but negative return should not be forgotten.

This bug caused "diff --no-index" to infinitely show the same error
message because the returned value was used to decrement the loop
control variable, e.g.

        $ git diff --no-index --color=words a b
        error: option `color' expects "always", "auto", or "never"
        error: option `color' expects "always", "auto", or "never"
        ...

Instead, make it act like so:

        $ git diff --no-index --color=words a b
        error: option `color' expects "always", "auto", or "never"
        fatal: invalid diff option/value: --color=words

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>