DirCache: Do not create duplicate tree entries

DirCache: Do not create duplicate tree entries

If a file (e.g.  "A") and a subtree file (e.g. "A/foo.c") both appear
in the DirCache this cache should not be written out as a tree object.
The "A" file and "A" subtree conflict with each other in the same tree
and will fail fsck.

Detect this condition during DirCacheBuilder and DirCacheEditor
finish() so the application can be halted early before it updates a
DirCache that might later write an invalid tree structure.

Change-Id: I95660787e88df336297949b383f4c5fda52e75f5