borg.sh: Drop unsupported and ineffective argument to realpath

The `realpath' on macOS (and likely elsewhere) lacks this argument.

We wanted to use a relative path so that `git clone' would store that
relative path in "<module>/.git"; turns out it converts any relative
path into an absolute path.

In other words, we lose nothing by dropping this argument.

Closes #167.

borg.sh

@@ -212,7 +212,7 @@ cmd_clone () {
         shift
     done
 
-    gitdir="$(realpath --relative-to=. "$(git rev-parse --git-dir)")"
+    gitdir="$(realpath "$(git rev-parse --git-dir)")"
 
     if [ $# -ne 0 ]
     then