Mark up file diffs that add, change or remove a symlink so that a label can be
shown in the UI to indicate that the file is a symlink.

Also normalize a git patch containing a symlink to make the file appear as a
regular file, otherwise patch fails to apply the diff, complaining about the
file not being a symlink.

Unit test.
Tested with a patch adding a symlink and changing another symlink. The files

were properly marked with " (symlink)" in the UI.