cli: use git config's `core.abbrev` for SHA-1 abbrevs
Implements radicle_cli::git::get_abbrev(), which will
get git's core.abbrev, which can be applied to COB IDs
and git OIDs (or any SHA1 abbreviation.)
This also respects special cases where core.abbrev can
be legally set to "auto" or "no", where "auto" gets an appropriate
length based on a loose count of objects in the current repo, and
"no" which doesn't abbreviate the SHA1 at all.