merge: make internal merge fail cleanly on symlinks
Simplemerge is not symlink aware and will never do the the right thing on
symlinks. It would read the symlink as a file and abort with 'No such file or
directory' on dangling symlinks.
Instead, internal:merge now simply fails to merge symlinks.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.