range-diff/format-patch: handle commit ranges other than A..B
In the `SPECIFYING RANGES` section of gitrevisions[7], two ways are
described to specify commit ranges that `range-diff` does not yet
accept: "<commit>^!" and "<commit>^-<n>".
Let's accept them, by parsing them via the revision machinery and
looking for at least one interesting and one uninteresting revision in
the resulting `pending` array.
This also finally lets us reject arguments that _do_ contain `..` but
are not actually ranges, e.g. `HEAD^{/do.. match this}`.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>