DeleteBranchCommand: update config only at the end

DeleteBranchCommand: update config only at the end

When multiple branches were to be removed, the git config was updated
after each and every branch. Newly do so only once at the end, after all
branches have been deleted.

Because there may be an exception after some branches have already been
deleted, take care to update the config even if an exception is thrown.

Bug: 451508
Change-Id: I645be8a1a59a1476d421e46933c3f7cbd0639fec
Signed-off-by: Thomas Wolf <twolf@apache.org>