happen to have both heads/master and tags/master, you can
explicitly say 'heads/master' to tell git which one you mean.
+* A suffix '@' followed by a date specification such as 'yesterday'
+ (24 hours ago) or '1 month 2 weeks 3 days 1 hour 1 second ago'
+ to specify the value of the ref at a prior point in time.
+ This suffix may only be used immediately following a ref name
+ and the ref must have an existing log ($GIT_DIR/logs/<ref>).
+
* A suffix '{caret}' to a revision parameter means the first parent of
that commit object. '{caret}<n>' means the <n>th parent (i.e.
'rev{caret}'