run_diff_files(): clarify computation of sha1 validity

Remove the need to have duplicated "if there is a change then feed
null_sha1 and otherwise sha1 from the cache entry" for the "new"
side of the diff by introducing two temporary variables to point
at the object name of the old and the new side of the blobs.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff-lib.c

@@ -101,6 +101,7 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 		struct cache_entry *ce = active_cache[i];
 		int changed;
 		unsigned dirty_submodule = 0;
+		const unsigned char *old_sha1, *new_sha1;
 
 		if (diff_can_quit_early(&revs->diffopt))
 			break;
@@ -224,9 +225,12 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 				continue;
 		}
 		oldmode = ce->ce_mode;
+		old_sha1 = ce->sha1;
+		new_sha1 = changed ? null_sha1 : ce->sha1;
 		diff_change(&revs->diffopt, oldmode, newmode,
-			    ce->sha1, (changed ? null_sha1 : ce->sha1),
-			    !is_null_sha1(ce->sha1), (changed ? 0 : !is_null_sha1(ce->sha1)),
+			    old_sha1, new_sha1,
+			    !is_null_sha1(old_sha1),
+			    !is_null_sha1(new_sha1),
 			    ce->name, 0, dirty_submodule);
 
 	}