Add `git lab -d` command

[kostajh]

trafficstars

Would be nice to support git lab -d {MR or URL}, as an equivalent to git review -d {changeId or URL}

[dancysoft]

From git review docs.

     -d change, --download=change
             Download change from Gerrit into a local branch. The branch will
             be named after the patch author and the name of a topic.  If the
             local branch already exists, it will attempt to update with the
             latest patchset for this change.

I personally use -x a lot:

    -x change, --cherrypick=change
             Apply change from Gerrit and commit into the current local branch
             ("cherry pick").  No additional branch is created.

             This makes it possible to review a change without creating a lo-
             cal branch for it. On the other hand, be aware: if you are not
             careful, this can easily result in additional patch sets for de-
             pendent changes. Also, if the current branch is different enough,
             the change may not apply at all or produce merge conflicts that
             need to be resolved by hand.