add a missing `git2::Status::WT_UNREADABLE` field

These codes are both already handled by Error::code(), but because
they weren't listed in Error::raw_code(), it would return -1, and so
the paths in Error::code() for them would never be hit.  This fixes
the code of errors from Repository::merge_commits() with
fail_on_conflict set for me.

Add missing code checks to Error::raw_code()

fix: add a backtick

Otherwise calling `Indexer::new` as the first thing in the program will
result in "no error" errors.

This updates the docs to clarify that the path is a *directory*.

Fix initialization with Indexer::new

Update version of git2 to 0.20.2