allow bad symlinks to survive a merge
Commands which perform merges will now install bad symlinks as symlinks
in the work tree, instead of creating them as regular files. This means
bad symlinks committed with 'got commit -S' (or Git) will be preserved.
The decision to introduce a bad symlink is done at commit-time and
merges should not forcefully reverse this decision.
The cherrypick and backout commands require a manual commit step, and
a merge result with bad symlinks will require use of 'got commit -S'.
Additional testing by thomas adam