Merge branch 'jc/merge-tag-object'

Merge branch 'jc/merge-tag-object'

"git merge $(git rev-parse v1.8.2)" behaved quite differently from
"git merge v1.8.2" as if v1.8.2 were written as v1.8.2^0 and did
not pay much attention to the annotated tag payload.

This makes the code notice the type of the tag object, in addition
to the dwim_ref() based classification the current code uses
(i.e. the name appears in refs/tags/) to decide when to special
case merging of tags.

* jc/merge-tag-object:
  t6200: test message for merging of an annotated tag
  t6200: use test_config/test_unconfig
  merge: a random object may not necssarily be a commit