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 enclosed in a brace
+ pair (e.g. '\{yesterday\}', '\{1 month 2 weeks 3 days 1 hour 1
+ second ago\}' or '\{1979-02-26 18:30:00\}') 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}'