apply: learn to use a different index file

Sometimes we want to apply in a different index file.

Before the apply functionality was libified it was possible to
use the GIT_INDEX_FILE environment variable, for this purpose.

But now, as the apply functionality has been libified, it should
be possible to do that in a libified way.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

apply.h

@@ -63,6 +63,7 @@ struct apply_state {
 	int unsafe_paths;
 
 	/* Other non boolean parameters */
+	const char *index_file;
 	enum apply_verbosity apply_verbosity;
 	const char *fake_ancestor;
 	const char *patch_input_file;