apply: handle empty patches

When a patch is empty, simply copy the source into the destination.

src/apply.c

@@ -262,7 +262,10 @@ int git_apply__patch(
 			patch->nfile.file->mode : GIT_FILEMODE_BLOB;
 	}
 
-	if ((error = apply_hunks(contents_out, source, source_len, patch)) < 0)
+	/* If the patch is empty, simply keep the source unchanged */
+	if (patch->hunks.size == 0)
+		git_buf_put(contents_out, source, source_len);
+	else if ((error = apply_hunks(contents_out, source, source_len, patch)) < 0)
 		goto done;
 
 	if (patch->delta->status == GIT_DELTA_DELETED &&

tests/apply/fromdiff.c

@@ -174,3 +174,11 @@ void test_apply_fromdiff__delete(void)
 		NULL, NULL,
 		PATCH_DELETE_ORIGINAL, NULL));
 }
+
+void test_apply_fromdiff__no_change(void)
+{
+	cl_git_pass(apply_buf(
+		FILE_ORIGINAL, "file.txt",
+		FILE_ORIGINAL, "file.txt",
+		"", NULL));
+}