This adds a brief help text to `git-subtree-check.sh` and adds and an
option to do a full remote check instead of having two different code
paths with a successful exit status. Also make it explicit that the CI
is not doing this.
git-subtree-check fails if the directory is given with a trailing slash,
eg:
```
> test/lint/git-subtree-check.sh src/univalue/
ERROR: src/univalue/ is not a subtree
```
Shell autocompletes will add the trailing slash when autofilling the
path name, which will therefore cause the script to fail.
Just ignore any trailing slash.
MarcoFalke