Allow upmerging repositories without a master branch

[acrobat]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tickets	Fix #...
License	MIT

In a repository where I use hubkit on, we don't have a master or main branch but only version branches.

So we have

• 5.10
• 6.0
• 6.1
• 6.x (which is the main branch where new versions are developed on)

So I've updated the code to include the default branch in list of upmerge branches if it is not already part of it. This way setups with a master default branch (or even the newer main branch) will still work but also support these kind of cases.

[sstok]

Thank you @acrobat