`git-commit-cancel': cancelling commits...

Since this is done by feeding git-commit(1) an empty commit message,
Magit will calmly notify us of its less-than-perfect exit status, which
is a minor inconvenience we might be willing to live with for now.

Signed-off-by: Pieter Praet <pieter@praet.org>
merge branch 'commit-cancel'
rename magit-commit{,-confirm}-{cancel => abort}

Rename `magit-commit-cancel' to `magit-commit-abort', and rename
`magit-commit-confirm-cancel' to `magit-commit-confirm-abort' for
consistency with `magit-rebase-abort'.