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>

org.eclipse.jgit/src/org/eclipse/jgit/dircache/DirCacheCheckout.java

@@ -304,29 +304,56 @@ public class DirCacheCheckout {
 	void processEntry(CanonicalTreeParser m, DirCacheBuildIterator i,
 			WorkingTreeIterator f) {
 		if (m != null) {
-			if (i == null || f == null || !m.idEqual(i)
-					|| (i.getDirCacheEntry() != null && (f.isModified(
-							i.getDirCacheEntry(), true) ||
-							i.getDirCacheEntry().getStage() != 0))) {
+			// There is an entry in the merge commit. Means: we want to update
+			// what's currently in the index and working-tree to that one
+			if (i == null) {
+				// The index entry is missing
 				update(m.getEntryPathString(), m.getEntryObjectId(),
 						m.getEntryFileMode());
+			} else if (f == null || !m.idEqual(i)) {
+				// The working tree file is missing or the merge content differs
+				// from index content
+				update(m.getEntryPathString(), m.getEntryObjectId(),
+						m.getEntryFileMode());
+			} else if (i.getDirCacheEntry() != null) {
+				// The index contains a file (and not a folder)
+				if (f.isModified(i.getDirCacheEntry(), true)
+						|| i.getDirCacheEntry().getStage() != 0)
+					// The working tree file is dirty or the index contains a
+					// conflict
+					update(m.getEntryPathString(), m.getEntryObjectId(),
+							m.getEntryFileMode());
+				else
+					keep(i.getDirCacheEntry());
 			} else
+				// The index contains a folder
 				keep(i.getDirCacheEntry());
 		} else {
+			// There is no entry in the merge commit. Means: we want to delete
+			// what's currently in the index and working tree
 			if (f != null) {
+				// There is a file/folder for that path in the working tree
 				if (walk.isDirectoryFileConflict()) {
 					conflicts.add(walk.getPathString());
 				} else {
+					// No file/folder conflict exists. All entries are files or
+					// all entries are folders
 					if (i != null) {
-						// ... and the working dir contained a file or folder ->
-						// add it to the removed set and remove it from
+						// ... and the working tree contained a file or folder
+						// -> add it to the removed set and remove it from
 						// conflicts set
 						remove(i.getEntryPathString());
 						conflicts.remove(i.getEntryPathString());
 					}
 				}
-			} else if (i.getDirCacheEntry().getStage() == 0)
-				keep(i.getDirCacheEntry());
+			} else {
+				// There is no file/folder for that path in the working tree.
+				// The only entry we have is the index entry. If that entry is a
+				// conflict simply remove it. Otherwise keep that entry in the
+				// index
+				if (i.getDirCacheEntry().getStage() == 0)
+					keep(i.getDirCacheEntry());
+			}
 		}
 	}