Get branch name once instead of repeatedly and incorrectly

The --branch parameter is used for a group of repos, so fetch it before iterating reposToClone. Fetching it inside the loop makes the clone operation correct only in the first iteration, after that --branch has been removed and branchName is empty.

See https://binary-factory.kde.org/job/Generate_API_Documentation/514/consoleText: only kfilemetadata uses kf5 branch, all other Frameworks repos use master.

Compare CMake command in the right sidebar on these 2 pages:

https://api.kde.org/frameworks/kfilemetadata/html/index.html

https://api.kde.org/frameworks/ki18n/html/index.html

@schwarzer @thiagosueto