Ambiguous ref names

Hi,

I'm re-posting this from the git-users Google Groups:

If I've two branches a.b.c and a.b.c.d, why does 'git checkout'
complains about ambiguous ref names? For example, if I'm in a.b.c.d,

896 $ git checkout a.b.c
warning: refname 'a.b.c' is ambiguous.
Switched to branch "a.b.c"

Is this some kind of a side effect of commit abbreviation? If so is
this kind of branch naming considered unsafe? Is there a way to make
git accept ref names verbatim, even if there's potential operation
that'd fail because of ambiguity?

-- 
:J