[REG] Branch dialog suggests local branch names that exist

(Cannot check atm, but believe the bug is present in 4.0 also)

This is directly related to QTCREATORBUG-15544, which added filtering to both local and remote branches.

Steps to reproduce:

• Have a repository with a local and remote branch "foo"
• The local branch must be inactive for some time, so it's not displayed when "Include old entries" is unchecked
• Select the remote branch foo, click Add, the Add Branch dialog suggests "foo" as local branch name
• The Version Control pane shows "cannot run git branch [...] a branch named 'foo' already exists."
• If include old entries is checked, the Add Branch dialog suggests "foo2" as local branch name, which works as expected