Fix warning when in detached HEAD state
Use of uninitialized value $existingBranch in scalar chomp at [...]/share/kdesrc-build/modules/ksb/Updater/Git.pm line 365.
Use of uninitialized value $existingBranch in scalar chomp at [...]/share/kdesrc-build/modules/ksb/Updater/Git.pm line 365.