[merge] Fix merge conflicts with symlinks

[merge] Fix merge conflicts with symlinks

Previous code would do a content merge on symlinks, and write the merge
result to the working tree as a file. C git doesn't do this; it leaves
a symlink in the working tree unchanged, or in a delete-modify conflict
it would check out "theirs".

Moreover, previous code would write the merge result to the link target,
not to the link. This would overwrite an existing link target, or fail
if the link pointed to a directory.

In link/file conflicts or file/link conflicts, C git always puts the
file into the working tree.

Change conflict handling accordingly. Add tests for all the conflict
cases.

Bug: 580347
Change-Id: I3cffcb4bcf8e336a85186031fff23f0c4b6ee19d
Signed-off-by: Thomas Wolf <twolf@apache.org>