commit-tree: update the command line parsing

We have kept the original "git commit-tree <tree> -p <parent> ..." syntax
forever, but "git commit-tree -p <parent> -p <parent> ... <tree>" would be
more intuitive way to spell it. Dashed flags along with their arguments
come first and then the "thing" argument after the flags.

Signed-off-by: Junio C Hamano <gitster@pobox.com>