[Bug] Porcelain allows creation of '@' branch

["Bence Csókás" <bence.csokas@arm.com>]

Hi,

I ran into this issue a few weeks ago. I'm using Git 2.55.0, which is
the latest released.

`git help check-ref-format` says this about a branch name:

   [...]
   9. They cannot be the single character @.
   [...]

And as expected, it is rejected:

   $ git check-ref-format @ && echo BUG!
   $

However, the following commands all create a branch named @ :

   $ git checkout -b @
   $ git switch -c @
   $ git branch @

I believe this to be a bug. Other invalid names are properly rejected
though:

   $ git checkout -b master@{1}
   fatal: 'master@{1}' is not a valid branch name
   hint: See 'git help check-ref-format'
   hint: Disable this message with "git config set advice.refSyntax false"
   $ git checkout -b @{1}
   fatal: '@{1}' is not a valid branch name
   hint: See 'git help check-ref-format'
   hint: Disable this message with "git config set advice.refSyntax false"
   $ git checkout -b @^
   fatal: '@^' is not a valid branch name
   hint: See 'git help check-ref-format'
   hint: Disable this message with "git config set advice.refSyntax false"

After creation, this branch cannot be checked out again, as `git
checkout @` is a no-op. Luckily though, `git branch -d @` works, so I
didn't permanently damage my Git repo :P

Bence

P.S. as I was typing this mail, I realized that I should've given
`--branch` to check-ref-format, and sure enough, there's the problem:

   $ git check-ref-format --branch @
   @
   $ git check-ref-format --branch @{1}
   fatal: '@{1}' is not a valid branch name
   $

Not sure why it thinks that would be a valid branch name...
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