Prevent unintended expansions in make_submodules fixture script

This quotes the command substitution so that if the current working
directory path contains IFS whitespace such as spaces, or contains
globbing characters, these will be treated literally, rather than
triggering word splitting or pathname expansion.

(Much less importantly, this also changes it to use the $() syntax
for command substitution. This is separate from quoting, since both
syntaxes need to be double-quoted to avoid splitting and globbing.)

Author: EliahKagan

gix/tests/fixtures/make_submodules.sh

@@ -160,7 +160,7 @@ mkdir symlinked-git-dir
     git commit -q --allow-empty -m "init"
   )
 
-  git config -f r1/.git/config core.worktree `pwd`
+  git config -f r1/.git/config core.worktree "$(pwd)"
   ln -s r1/.git .git
 
   git -c protocol.file.allow=always submodule add ../module1 m1