git revert --reference doesn't populate commit subject

[Bagas Sanjaya <bagasdotme@gmail.com>]

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Hi,

When reverting commit with `git revert`, the subject for revert commit
is populated ("Revert <original commit subject>"). However, when
reverting with `git revert --reference`, the subject doesn't get
populated; only commit message is generated ("This reverts <original
commit reference>..."). When no subject is given, the first paragraph of
the message automatically becomes commit subject.

To reproduce:

```
$ mkdir /tmp/test && cd /tmp/test
$ git init && git branch -m main
$ echo "test" > test
$ git add test && git commit -m "test"
$ echo "whoops" > test
$ git commit -a -m "whoops"
$ git revert HEAD
// keep the commit message by just exiting the editor
$ git log -n 1 --pretty="format:%s" -p HEAD
  Revert "whoops"
$ echo "whoops" > test
$ git commit -a -m "whoops again"
$ git revert --reference HEAD
// keep the commit message by just exiting the editor
$ git log -n 1 --pretty="format:%s" -p HEAD
  This reverts <reference to "whoops again" commit>.
```

In the reproducer above, I expect that the second revert generates
`Revert "whoops again"` subject.

Thanks.

-- 
An old man doll... just what I always wanted! - Clara

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