Re: tracking branch for a rebase

Jeff King [off-list ref] writes:

I wonder if it is worth adding @{upstream} now, which is fairly safe,
letting it cook for a while, and then adding a "%" alias later after the
concept has proved itself (and people say "I like this feature, but it
really is too much to type").

That's a sane suggestion, I think.  We can always have a descriptive
longhand (e.g. "branch@{upstream}") and later add a shorthand for often
used ones (e.g. "branch^up", "branch/.up" --- the former is possible
because it cannot be upth parent of the commit at the tip of the branch,
and the latter is possible because component of hierarchical refname
cannot begin with a dot).

I find a prefix % not descriptive enough (besides being ugly); if it were
"^branch", as some people said, it would probably have matched its meaning
"up", but that notation is already taken for "uninteresting".  If we are
going to use funny symbol as a notation to invoke magic, I think it is
easier for new people if we limit the number of symbols used.  We already
have ^n, ^{type} and ~n magic, all of which operate on objects and peel
them in three different ways.

Then there currently is only one kind of magic that works on refs and it
is spelled as ref@{magic}.  Let's try not to introduce more notation
before it is absolutely necessary.

When I say there is only one kind of magic notation for refs, I am
primarily talking about the end-user perception.  @{time}, @{number} and
@{-number} all do their magic using the reflog, but that is about _how_
they do what they do.  End-user perception begins with _what_ they do, and
at that level, the magic consistently works on refs and different genie is
summoned depending on what is inside {}.

The @{upstream} thing won't be using reflog to do its job, but that is
about _how_ it is implemented, and the end users don't care.

What is more important is _what_ it does.  Given a ref, the @{} magic
notation finds something that is related to the ref.  @{time} and
@{number} finds what the ref itself used to point at. @{-number}
(applicable only to HEAD ref) finds the ref it used to point at.  And
@{upstream} will find another ref that it merges from (or rebases onto).