Fixed Grit::Git#options_to_argv generates invalid sequence of arguments.

When mixed short-style option and long-style option, 
options_to_argv often generates invalid sequence of arguments.

  repo.git.branch(:merged => true, :a => true)

In this case, options_to_argv should generates "-a --merged", 
but "--merged -a".
Then it execute command like "git branch --merged -a", it will
happen 'fatal: malformed object name -a'.

Arguments that generated by options_to_argv should be sorted by option's
length.
For examples, "-a" option should be in front of "--no-merged" option.