Re: [PATCH 1/2] sha1_name.c: signal if @{-N} was a true branch nameor a detached head

> The original API read "checkout: moving from (.*) to ..." from the
> reflog of the HEAD, and returned the substring between "from" and
> "to", but there was no way, if the substring was a 40-hex string, to
> tell if we were on a detached HEAD at that commit object, or on a
> branch whose name happened to be the 40-hex string.
>
> At this point, we cannot afford to change the format recorded in the
> reflog, so introduce a heuristics to see if the 40-hex matches the
> object name of the commit we are switching out of.  This will
> unfortunately mishandle this case:
>
>         HEX=$(git rev-parse master)
>         git checkout -b $HEX master
>         git checkout master
>
> where we were indeed on a non-detached $HEX branch (i.e. HEAD was
> pointing at refs/heads/$HEX, not storing $HEX), of course, but
> otherwise should be fairly reliable.
>
> Signed-off-by: Junio C Hamano <gitster@pobox.com>