builtin/describe.c: factor out describe_commit

builtin/describe.c: factor out describe_commit

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>