Using `coo` to open a branch under the cursor that started with 4+ hex
digits (e.g., 1111-feat) would fail with the error `fatal: invalid
reference: 1111.`
This happened because the prior regex would consider these branch names
as commits when determining whether to treat what was under the cursor
as a branch or commit.
This changes the regex to use a negative look-ahead to further tighten
the match for a commit and require that a match on 4+ hex digits also
not end with a non-hex digit.
Resolves#2085.
