scalar: accept -C and -c options before the subcommand

scalar: accept -C and -c options before the subcommand

The `git` executable has these two very useful options:

-C <directory>:
	switch to the specified directory before performing any actions

-c <key>=<value>:
	temporarily configure this setting for the duration of the
	specified scalar subcommand

With this commit, we teach the `scalar` executable the same trick.

Note: It might look like a good idea to try to reuse the
`handle_options()` function in `git.c` instead of replicating only the
`-c`/`-C` part. However, that function is not only not in `libgit.a`, it
is also intricately entangled with the rest of the code in `git.c` that
is necessary e.g. to handle `--paginate`. Besides, no other option
handled by that `handle_options()` function is relevant to Scalar,
therefore the cost of refactoring vastly would outweigh the benefit.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>