acceptance: guard git-repo-init against running inside an existing repo (#5281)

## Why

"git config" writes to whatever repo CWD is in, so calling git-repo-init
from inside an existing repo poisons that repo's config without any
error signal. I had my cli repo config messed up by an agent.

Co-authored-by: Isaac

Author: denik

acceptance/script.prepare

@@ -55,6 +55,10 @@ trace() {
 }
 
 git-repo-init() {
+    if git rev-parse --git-dir > /dev/null 2>&1; then
+        echo "git-repo-init: already inside a git repository" >&2
+        return 1
+    fi
     git init -qb main
     git config core.autocrlf false
     git config user.name "Tester"