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.c

@@ -1735,12 +1735,23 @@ void grep_source_init(struct grep_source *gs, enum grep_source_type type,
 	case GREP_SOURCE_FILE:
 		gs->identifier = xstrdup(identifier);
 		break;
+	case GREP_SOURCE_SUBMODULE:
+		if (!identifier) {
+			gs->identifier = NULL;
+			break;
+		}
+		/*
+		 * FALL THROUGH
+		 * If the identifier is non-NULL (in the submodule case) it
+		 * will be a SHA1 that needs to be copied.
+		 */
 	case GREP_SOURCE_SHA1:
 		gs->identifier = xmalloc(20);
 		hashcpy(gs->identifier, identifier);
 		break;
 	case GREP_SOURCE_BUF:
 		gs->identifier = NULL;
+		break;
 	}
 }
 
@@ -1760,6 +1771,7 @@ void grep_source_clear_data(struct grep_source *gs)
 	switch (gs->type) {
 	case GREP_SOURCE_FILE:
 	case GREP_SOURCE_SHA1:
+	case GREP_SOURCE_SUBMODULE:
 		free(gs->buf);
 		gs->buf = NULL;
 		gs->size = 0;
@@ -1831,8 +1843,10 @@ static int grep_source_load(struct grep_source *gs)
 		return grep_source_load_sha1(gs);
 	case GREP_SOURCE_BUF:
 		return gs->buf ? 0 : -1;
+	case GREP_SOURCE_SUBMODULE:
+		break;
 	}
-	die("BUG: invalid grep_source type");
+	die("BUG: invalid grep_source type to load");
 }
 
 void grep_source_load_driver(struct grep_source *gs)

grep.h

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