Support for symlinks in compare editor
This change makes compare editor works same way the git diff works for
symlinks: it shows *not* the diff between the file or folder contents,
but diff for the symlink target paths.
The compare editor gives a convenient way (given both bug 484498 and bug
484491 patches are merged) to modify symlink target in the IDE, without
using command line tooling.
Bug: 484494
Change-Id: I58d2a5d969886fc00e5aee8f7b83ac57cf6cfa7f
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>