update "all" includes only modules updated on master

This one is inconvenient mostly from a cosmetic perspective, but the fix is trivial. When running `update -a` _on a release_ branch, the command compares the current submodule HEADs to the corresponding HEADs on the _master_ branch. In other words, any submodule that has progressed on the master branch will be included in an update on the repository release branch.

1. It's almost always the case that the intended submodules will be updated on the release branch because progression on the submodule _release_ branch would be preceded by progression on the master branch.
2. The cosmetic implication is that the commit message for the update will list every module that has progressed on the _master_ branch. When the update attempts to commit submodules that have not actually progressed on the release branch, the command will be a noop and will succeed quietly.

All of this is simply because of [line 48](https://github.com/preaction/Git-ReleaseRepo/blob/master/lib/Git/ReleaseRepo/Command/update.pm#L48) in Git::ReleaseRepo::Command::update:

```perl
$args = [$git->outdated_branch];
```

Instead the invocation of `outdated_branch` needs to specify the current branch on which the update is being run.

```perl
$args = [$git->outdated_branch( $branch )];
```

I've submitted a pull request with a test and patch. The patch additionally introduces handling in the `update` command for potential errors during the commit.