Merge branch 'tc/last-modified-not-a-tree'

Merge branch 'tc/last-modified-not-a-tree'

Giving "git last-modified" a tree (not a commit-ish) died an
uncontrolled death, which has been corrected.

* tc/last-modified-not-a-tree:
  last-modified: verify revision argument is a commit-ish
  last-modified: remove double error message
  last-modified: fix memory leak when more than one commit is given
  last-modified: rewrite error message when more than one commit given