Refactor and comment complicated if statements

Refactor and comment complicated if statements

When debugging and enhancing DirCacheCheckout.processEntry() I found
that some of if-statements where hard to read/understand. This
change just splits some long if statements and adds more comments
explaining in which state we are. This change is only a preparation
for followup commits which introduce checks for untracked+ignored
files.

Change-Id: I670ff08310b72c858709b9e395f0aebb4b290a56
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>