Move "--add=<branch>" validity check out of argument loop

This will allow us to use values given as command-line arguments when
validating branch names.

Signed-off-by: John Keeping <john@keeping.me.uk>
Respect prefix when checking branch given to --add

Otherwise the command may erroneously report that the branch is not
found and fail.

Signed-off-by: John Keeping <john@keeping.me.uk>
Merge branch 'add-respect-prefix' into pu

* branch add-respect-prefix:
  Respect prefix when checking branch given to --add
  Move "--add=<branch>" validity check out of argument loop

Fix prefix handling when validating arguments to --add.