planemo shed_update --help does not explain return codes

e.g. With an actual change to a file:

```
$ emacs tools/mira3/README.rst
$ git commit -m "Update copyright year" tools/mira3/README.rst
[master 8423277] Update copyright year
1 files changed, 1 insertions(+), 1 deletions(-)
[galaxy@ppserver galaxy_mira]$ planemo shed_update -t testtoolshed --fail_fast --force_repository_creation tools/mira3 ; echo "Returned $?"
cd '/tmp/galaxy_mira/tools/mira3' && git rev-parse HEAD
cd '/tmp/galaxy_mira/tools/mira3' && git diff --quiet
Repository mira_assembler updated successfully.
Repository metadata updated.
Returned 0
```

I'm guessing zero/success means an update was done?

Here I attempted to change just the meta-data:

```
$ emacs tools/mira3/.shed.yml
$ git commit -m "Suggest using MIRA 4 onwards" tools/mira3/.shed.yml
[master ea6742e] Suggest using MIRA 4 onwards
$ git commit -m "Suggest using MIRA 4 onwards" tools/mira3/.shed.yml
[master ea6742e] Suggest using MIRA 4 onwards
1 files changed, 1 insertions(+), 1 deletions(-)
[galaxy@ppserver galaxy_mira]$ planemo shed_update -t testtoolshed --fail_fast --force_repository_creation tools/mira3 ; echo "Returned $?"
cd '/tmp/galaxy_mira/tools/mira3' && git rev-parse HEAD
cd '/tmp/galaxy_mira/tools/mira3' && git diff --quiet
Could not update mira_assembler
Unexpected response from galaxy: 400: {"content_alert": "", "err_msg": "No changes to repository."}
Repository metadata updated.
Failed to update a repository.
Returned 255
```

Here I changed nothing:

```
$ planemo shed_update -t testtoolshed --fail_fast --force_repository_creation tools/mira3 ; echo "Returned $?"
cd '/tmp/galaxy_mira/tools/mira3' && git rev-parse HEAD
cd '/tmp/galaxy_mira/tools/mira3' && git diff --quiet
Could not update mira_assembler
Unexpected response from galaxy: 400: {"content_alert": "", "err_msg": "No changes to repository."}
Repository metadata updated.
Failed to update a repository.
Returned 255
```

It is unclear here from the output & return code if the metadata update actually applied any changes or not... but the none zero return code of 255 presumably means no files changed?