Make sure not to overwrite untracked not-ignored files

Make sure not to overwrite untracked not-ignored files

When DirCacheCheckout was checking out it was silently
overwriting untracked files. This is only ok if the
files are also ignored. Untracked and not ignored files
should not be overwritten. This fix adds checks for
this situation.
Because this change in the behaviour also broke tests
which expected that a checkout will overwrite untracked
files (PullCommandTest) these tests have to be modified
also.

Bug: 333093
Change-Id: I26806d2108ceb64c51abaa877e11b584bf527fc9
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>