Factor out describing commits into its own function `describe_commit`,
which will put any output to stdout into a strbuf, to be printed
afterwards.
As the next patch will teach Git to describe blobs using a commit and path,
this refactor will make it easy to reuse the code describing commits.
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>