Merge branch 'ij/subtree-reject-v2-config' into jch

Merge branch 'ij/subtree-reject-v2-config' into jch

The shell script implementation of 'git subtree' has been updated to
check for the presence of the configuration file of the new Rust
implementation, preventing users from accidentally running the old
script on repositories already managed by the new tool.

* ij/subtree-reject-v2-config:
  git-subtree: Bail out if we find output from Rust rewrite (test)
  git-subtree: Bail out if we find output from Rust rewrite