apply: split load_preimage() helper function out

Given a patch for a single path, the function apply_data() reads the
preimage in core, and applies the change represented in the patch.

Separate out the first part that reads the preimage into a separate
helper function load_preimage().
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/apply.c

@@ -3046,10 +3046,10 @@ static int checkout_target(struct cache_entry *ce, struct stat *st)
 	return 0;
 }
 
-static int apply_data(struct patch *patch, struct stat *st, struct cache_entry *ce)
+static int load_preimage(struct image *image,
+			 struct patch *patch, struct stat *st, struct cache_entry *ce)
 {
 	struct strbuf buf = STRBUF_INIT;
-	struct image image;
 	size_t len;
 	char *img;
 	struct patch *tpatch;
@@ -3086,7 +3086,16 @@ static int apply_data(struct patch *patch, struct stat *st, struct cache_entry *
 	}
 
 	img = strbuf_detach(&buf, &len);
-	prepare_image(&image, img, len, !patch->is_binary);
+	prepare_image(image, img, len, !patch->is_binary);
+	return 0;
+}
+
+static int apply_data(struct patch *patch, struct stat *st, struct cache_entry *ce)
+{
+	struct image image;
+
+	if (load_preimage(&image, patch, st, ce) < 0)
+		return -1;
 
 	if (apply_fragments(&image, patch) < 0)
 		return -1; /* note with --reject this succeeds. */