Installs and removes the git hooks that run mix presubmit.

Two hooks work together so that the change set checked is the commit that is about to exist:

• prepare-commit-msg learns from git whether the commit amends HEAD (source commit with HEAD's SHA) and records HEAD^ as the base in .git/presubmit_base, or removes that file otherwise.
• commit-msg runs mix presubmit --staged --message-file "$1", adding --base <recorded> when amending, so rules see HEAD^ → index — the amended commit — rather than the delta since HEAD, which could never satisfy a rule whose other half lives in the original commit.

Git reports the message source as message rather than commit when --amend is combined with -m or -F, so that form of amend is not detected and the delta since HEAD is what gets checked; amend with the editor or --no-edit, or run mix presubmit --staged --base HEAD^.

A hook blocks the commit only on presubmit's verdict. It points PRESUBMIT_VERDICT at a file under the git directory, and mix presubmit writes pass or fail there once its rules have run (Presubmit.CLI). When there is no verdict, presubmit did not run: Mix stopped first (stale or unresolvable dependencies, a project that does not load), or presubmit hit a configuration error or crashed. That is a fault in the checkout or the tool, not in the commit, so the hook prints presubmit could not run: <the error>; commit allowed, CI still checks and lets the commit through. Presubmit 0.1.0 writes no verdict; its report (a line starting Examining) shows that it ran, and then its exit status is the verdict.

The hooks also step aside, with a note, while a merge is in progress or when amending a merge commit (the change set would be someone else's branch), and when mix is not on PATH (GUI clients often lack the developer's shell environment). A rule that crashes is reported but does not block the commit (--on-error warn). CI remains the backstop for all of these.

Hooks are installed for the project in the current directory; in a subdirectory project of a larger repository the scripts cd there first. A hook that was not installed by this module is never overwritten, and installation is refused when core.hooksPath points hooks elsewhere.