tools/workflow: Fix rebase command in close()

Hotfix for issue #35.
tools/worflow: Print important git commands executed

If we're not on a dry run, print the git commands that are executed
which don't provide obvious feedback. For example:

    $ ./tools/workflow new 10
    Switched to a new branch 'wip/10'
    git push -u origin wip/10
    Counting objects: 59, done.
    Compressing objects: 100% (53/53), done.
    Writing objects: 100% (54/54), 6.50 KiB, done.
    Total 54 (delta 38), reused 0 (delta 0)
    To git@github.com:ChrisCummins/protein-db.git
     * [new branch]      wip/10 -> wip/10
    Branch wip/10 set up to track remote branch wip/10 from origin.

    Execute './workflow close' when completed.
    $ echo 'my hotfox' > fix
    $ git commit -a -m 'Fix for #10'
    [wip/10 672b654] Fix for #10
     1 file changed, 1 insertion(+)
     create mode 100644 tools/fix
    $ ./workflow close
    Switched to branch 'master'
    Your branch is ahead of 'origin/master' by 1 commit.
    git rebase wip/10
    First, rewinding head to replay your work on top of it...
    Fast-forwarded master to wip/10.
    Deleted branch wip/10 (was 672b654).
    To git@github.com:ChrisCummins/protein-db.git
     - [deleted]         wip/10

    Merged 'wip/10' into 'master'

Issue #34.