name-rev: factor code for sharing with a new command

name-rev: factor code for sharing with a new command

We are about to introduce a new command git-format-rev(1) to this
file. Let’s factor some code so that we can share it with the new
command.

We want to be able to format commits found in freeform text, and
git-name-rev(1) already has a function for that but for symbolic
names. Let’s use a tagged union for the command-specific payload.

No functional changes.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>