There's been a recent movement to rename the "master" branch in git
repositories to "main" to avoid to potentially problematic word
"master". Our `merge-branch` script was hard coded to "master". This PR
makes it possible to pass in the name of the branch you wish to merge
to. I've defaulted it to "master" for now, but I expect in time this
will change.
This change prevents chained commands from continuing execution if the
`git merge` is operation would be unsuccessful such as a common
operation when a feature branch is ready to be merged and pushed:
```
git merge-branch && git push origin
```
If the origin/master and the local master have any difference, we return
status 1 and stop execution rather then emit a false success.
h/t: @croaky
Fix#563.