Set LC_ALL=C when using sed on a binary file

Because some sed implementations, at least the one on macOS, detect
invalid text in the current locale's encoding and error out. See:

https://stackoverflow.com/questions/19242275/re-error-illegal-byte-sequence-on-mac-os-x

This makes the script work on macOS.

Author: EliahKagan

gix-worktree/tests/fixtures/make_traverse_dotdot_trees.sh

@@ -29,7 +29,7 @@ for subdir in .a .b .c .d .e .f .g .h .i .j; do
 
     standin_pattern="$(printf '%s' "$standin" | sed 's|\.|\\\.|g')"
     cp .git/index old_index
-    sed "s|$standin_pattern|$target|g" old_index >.git/index
+    LC_ALL=C sed "s|$standin_pattern|$target|g" old_index >.git/index
 done
 
 git commit -m 'Initial commit'