Merge branch 'sg/describe-contains'

"git describe" without argument defaulted to describe the HEAD
commit, but "git describe --contains" didn't.  Arguably, in a
repository used for active development, such defaulting would not
be very useful as the tip of branch is typically not tagged, but it
is better to be consistent.

* sg/describe-contains:
describe --contains: default to HEAD when no commit-ish is given

diff --cc Documentation/git-describe.txt
Simple merge

diff --cc builtin/describe.c
Simple merge