parse_long_opt always matches both --opt and --no-opt for any option
"opt", and only get_value checks whether --no-opt is actually valid.
Since the options for git branch contains both "no-merged" and "merged"
there are two matches for --no-merge, but no exact match.  With this
patch the negation of a NONEG option is rejected earlier, but it changes
the error message from "option `no-opt' isn't available" to "unknown
option `no-opt'".

[jk: added test]
Signed-off-by: NAndreas Schwab <schwab@linux-m68k.org>
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>