Re: [PATCH 1/3] range-diff: refactor check for commit range

"Johannes Schindelin via GitGitGadget" [off-list ref]
writes:

From: Johannes Schindelin <redacted>

Currently, when called with exactly two arguments, we test for a literal
`..` in each of the two.

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 conditional into its own function.

Makes sense.