grep: add submodules as a grep source type

Add `GREP_SOURCE_SUBMODULE` as a grep_source type and cases for this new
type in the various switch statements in grep.c.

When initializing a grep_source with type `GREP_SOURCE_SUBMODULE` the
identifier can either be NULL (to indicate that the working tree will be
used) or a SHA1 (the REV of the submodule to be grep'd).  If the
identifier is a SHA1 then we want to fall through to the
`GREP_SOURCE_SHA1` case to handle the copying of the SHA1.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.h

@@ -161,6 +161,7 @@ struct grep_source {
 		GREP_SOURCE_SHA1,
 		GREP_SOURCE_FILE,
 		GREP_SOURCE_BUF,
+		GREP_SOURCE_SUBMODULE,
 	} type;
 	void *identifier;