Currently, when called with exactly two arguments, `git range-diff`
tests for a literal `..` in each of the two. Likewise, the argument
provided via `--range-diff` to `git format-patch` is checked in the same
manner.

However, `<commit>^!` is a perfectly valid commit range, equivalent to
`<commit>^..<commit>` according to the `SPECIFYING RANGES` section of
gitrevisions[7].

In preparation for allowing more sophisticated ways to specify commit
ranges, let's refactor the check into its own function.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

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>

There are three forms, depending whether the user specifies one, two or
three non-option arguments. We've never actually explained how this
works in the manual, so let's explain it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>