On many keyboards, typing "@{" involves holding down SHIFT key and
one can easily end up with "@{Up..." when typing "@{upstream}". As
the upstream/push keywords do not appear anywhere else in the syntax,
we can safely accept them case insensitively without introducing
ambiguity or confusion to solve this.
* ab/case-insensitive-upstream-and-push-marker:
rev-parse: match @{upstream}, @{u} and @{push} case-insensitively