Add patch to fix spelling of error message

This is fixed upstream but lintian isn't happy

debian/patches/fix_spelling.patch

@@ -0,0 +1,14 @@
+Fix spelling of error string.
+diff --git a/src/tree.c b/src/tree.c
+index 9655ad7..7afef92 100644
+--- a/src/tree.c
++++ b/src/tree.c
+@@ -1257,7 +1257,7 @@ int git_tree_create_updated(git_oid *out, git_repository *repo, git_tree *baseli
+ 				break;
+ 			}
+ 			default:
+-				giterr_set(GITERR_TREE, "unkown action for update");
++				giterr_set(GITERR_TREE, "unknown action for update");
+ 				error = -1;
+ 				goto cleanup;
+ 		}

debian/patches/series

@@ -1,2 +1,3 @@
 disable_tests.patch
 fix_https_test.patch
+fix_spelling.patch