Merge branch 'jc/allow-ff-merging-kept-tags' into jch

Merge branch 'jc/allow-ff-merging-kept-tags' into jch

Since Git 1.7.9, "git merge" defaulted to --no-ff (i.e. even when
the side branch being merged is a descendant of the current commit,
create a merge commit instead of fast-forwarding) when merging a
tag object.  This was appropriate default for integrators who pull
signed tags from their downstream contributors, but caused an
unnecessary merges when used by downstream contributors who
habitually "catch up" their topic branches with tagged releases
from the upstream.  Update "git merge" to default to --no-ff only
when merging a tag object that does *not* sit at its usual place in
refs/tags/ hierarchy, and allow fast-forwarding otherwise, to
mitigate the problem.

* jc/allow-ff-merging-kept-tags:
  merge: allow fast-forward when merging a tracked tag