User attrs start by @ to avoid semantics' breakage

To avoid semantics' breakage, as said in Git development list [1,2],
user attributes now start by '@', so an attribute that does not
start by '@' is considered an 'official' attribute (has semantics)
and an attribute that starts with '@' is considered an user's
attribute.

[1] http://thread.gmane.org/gmane.comp.version-control.git/138848/focus=138892
[2] http://thread.gmane.org/gmane.comp.version-control.git/19126/focus=19149