rebase: clarify "restore the original branch"

rebase: clarify "restore the original branch"

The description for 'git rebase --abort' currently says:

    Restore the original branch and abort the rebase operation.

The "restore" can be misinterpreted to imply that the original branch
was somehow in a broken state during the rebase operation. It is also
not completely clear what "the original branch" is --- is it the
branch that was checked out before the rebase operation was called or
is the the branch that is being rebased (it is the latter)? Although
both issues are made clear in the DESCRIPTION section, let us also
make the entry in the OPTIONS secion more clear.

Also remove the term "rebasing process" from the usage text, since the
user already knows that the text is about "git rebase".

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>