grep: convert to struct object_id

Convert the remaining parts of grep to use struct object_id.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/grep.h b/grep.h
index 267534c..c88b40b 100644
--- a/grep.h
+++ b/grep.h
@@ -158,7 +158,7 @@ struct grep_source {
 	char *name;
 
 	enum grep_source_type {
-		GREP_SOURCE_SHA1,
+		GREP_SOURCE_OID,
 		GREP_SOURCE_FILE,
 		GREP_SOURCE_BUF,
 		GREP_SOURCE_SUBMODULE,