Introduce metaData compare between working tree and index entries

Introduce metaData compare between working tree and index entries

Instead of offering only a high-level isModified() method a new
method compareMetadata() is introduced which compares a working tree entry
and a index entry by looking at metadata only. Some use-cases
(e.g. computing the content-id in idBuffer()) may use this new method
instead of isModified().

Change-Id: I4de7501d159889fbac5ae6951f4fef8340461b47
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>