diff_util: Add the short alias S for --stat

[PhilipMetzger]

This was a suggestion from sm in the libera #jujutsu IRC chat.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added tests to cover my changes

[simonmichael]

More context, in case it helps:

what would you think of adding -S as short flag for --stat (for jj diff, eg)
"like -s, but louder"
the value could be having a discoverable convenient shortcut for all.. if it's not too clashy

I was repeatedly running diff (or log) with successively more detail:

jj diff, jj diff -s, jj diff --stat, jj diff -p
and using terminal history to edit the previous command. The --stat is just a little awkward to type each time. 

[martinvonz]

There are only ~50 possible short flags, so we need to be careful about adding new ones. I don't think we've heard the request often enough that we should add a flag.

[PhilipMetzger]

There are only ~50 possible short flags, so we need to be care. I don't think we've heard the request often enough that we should add a flag.

Agreed, so I'll leave it up for @simonmichael until we hear more demand for it,

[simonmichael]

Waiting to see if this is worthwhile is quite understandable.