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>