Update doMirrorUpdate to be able to pass a branch name to it. Pass an
empty branch when we want to deploy the whole thing, and pass only a
specific branch when `-branch=...` has been specified. `git fetch` does
not lock the whole index and only pulls down the branch which avoids the
whole plethora of issues when more than one instance of g10k is running.
For example, we could run into issues like this when `git remote update
--prune` is running more than once:

error: Ref refs/heads/test is at 1d793f181f3a460916ce1ab5dc33e007622a9a61 but expected c28d866fdfed5be79f5ea453e04ce887627a8efa
! c28d866..1d793f1  test -> test (unable to update local ref)

Also, avoiding updating the whole index means that the performance
significantly improves. In my testing the whole time it takes to deploy
a branch has been reduced by about 10 seconds.

Update executeCommand() to leave output intact by separating two
different concerns: the error and the output itself. Still always set
the returnCode to 1 if any error has occurred.

`timeout` is passed now down to executeCommand() but nothing is done
with it. Properly make a context and pass it to exec.CommandContext().
Also, while at it add proper debug printing so that output messages
*and* the error would appear in the console if `-debug=true` is
specified.

Otherwise we might run into an error like this:

From https://git.foo.bar/lol.git
 ! [rejected]        dimstoperms -> dimstoperms  (non-fast-forward)