Make partial clone imply no-clone-bundle by default.

For large projects, clone bundle is useful because it provided a way to
efficiently transfer a large portion of git objects through CDN, without
needing to interact with git server. However, with partial clones, the
intention is to not download most of the objects, so the use of clone
bundles would defeat the space savings normally seen with partial
clones, as they are downloaded before the first fetch.

A new option, --clone-bundle is added to override this behavior.
Add a new repo.clonebundle variable which remembers the choice if
explicitly given from command line at repo init.

Change-Id: I03638474af303a82af34579e16cd4700690b5f43
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/268452
Tested-by: Xin Li <delphij@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>