Branch should default to master when no ref is available

[nmattia]

Reproduce with:

/tmp/tmp.lFe9UOQv1Y$ niv add git git://git.ipxe.org/ipxe.git
Adding package ipxe
  FATAL: Could not read the output of 'git'.
  command: git ls-remote --symref git://git.ipxe.org/ipxe.git HEAD
  Could not read reference and revision from stdout:
  3fe683ebab29afacf224e6b0921f6329bebcdca7	HEAD
  
  
  This is a bug. Please create a ticket:
    https://github.com/nmattia/niv/issues/new
  Thanks! I'll buy you a beer.
  FATAL: One or more packages failed to update:
  ipxe: ExitFailure 1
ERROR: ExitFailure 1