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>

contrib/scalar/t/t9099-scalar.sh

@@ -85,4 +85,12 @@ test_expect_success 'scalar delete with enlistment' '
 	test_path_is_missing cloned
 '
 
+test_expect_success 'scalar supports -c/-C' '
+	test_when_finished "scalar delete sub" &&
+	git init sub &&
+	scalar -C sub -c status.aheadBehind=bogus register &&
+	test -z "$(git -C sub config --local status.aheadBehind)" &&
+	test true = "$(git -C sub config core.preloadIndex)"
+'
+
 test_done