add-patch: classify '@' as a synonym for 'HEAD'

Currently, (restore, checkout, reset) commands correctly take '@' as a
synonym for 'HEAD'. However, in patch mode different prompts/messages
are given on command line due to patch mode machinery not considering
'@' to be a synonym for 'HEAD' due to literal string comparison with
the word 'HEAD', and therefore assigning patch_mode_($command)_nothead
and triggering reverse mode (-R in diff-index). The NEEDSWORK comment
suggested comparing commit objects to get around this. However, doing
so would also take a non-checked out branch pointing to the same commit
as HEAD, as HEAD. This would cause confusion to the user.

Therefore, after parsing '@', replace it with 'HEAD' as reasonably
early as possible. This also solves another problem of disparity
between 'git checkout HEAD' and 'git checkout @' (latter detaches at
the HEAD commit and the former does not).

Trade-offs:
- Some of the errors would show the revision argument as 'HEAD' when
  given '@'. This should be fine, as most users who probably use '@'
  would be aware that it is a shortcut for 'HEAD' and most probably
  used to use 'HEAD'. There is also relevant documentation in
  'gitrevisions' manpage about '@' being the shortcut for 'HEAD'. Also,
  the simplicity of the solution far outweighs this cost.

- Consider '@' as a shortcut for 'HEAD' even if 'refs/heads/@' exists
  at a different commit. Naming a branch '@' is an obvious foot-gun and
  many existing commands already take '@' for 'HEAD' even if
  'refs/heads/@' exists at a different commit or does not exist at all
  (e.g. 'git log @', 'git push origin @' etc.). Therefore this is an
  existing assumption and should not be a problem.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Phillip Wood <phillip.wood123@gmail.com>
Signed-off-by: Ghanshyam Thakkar <shyamthakkar001@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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