$ git-tag -s -m test test     
gpg: skipped "Sir Raorn <raorn@...>": secret key not available
gpg: signing failed: secret key not available
failed to sign the tag with GPG.
[1]    28905 exit 1     git-tag -s -m test test

$ expr "z$(git-var GIT_COMMITTER_IDENT)" : 'z\(.*>\)'       
Sir Raorn <raorn@...>

$ gpg --list-secret-keys 'Sir Raorn <raorn@..>'        
gpg: error reading key: secret key not available
[1]    28931 exit 2     gpg --list-secret-keys 'Sir Raorn <raorn@..>'

$ gpg --list-secret-keys 'Alexey I. Froloff <raorn@..>'
gpg: error reading key: secret key not available
[1]    28933 exit 2     gpg --list-secret-keys 'Alexey I. Froloff <raorn@..>'

$ gpg --list-secret-keys 'Alexey I. Froloff (Sir Raorn) <raorn@..>'            
                            
...
uid                  Alexey I. Froloff (Sir Raorn) <raorn@..>
...


Patch attached.