Merge branch 'hd/show-one-mergetag-fix'

"git show" and others gave an object name in raw format in its
error output, which has been corrected to give it in hex.

* hd/show-one-mergetag-fix:
  show_one_mergetag: print non-parent in hex form.

diff --cc log-tree.c
Simple merge

diff --cc t/t4202-log.sh
Simple merge