Refactored code out of FileHeader to facilitate rename detection

Refactored a superclass out of FileHeader called DiffEntry that holds
the more general data from FileHeader that is useful in rename
detection (old/new Ids, modes, names, as well as changeType and
score). FileHeader is now a DiffEntry that adds Hunks, parsing
abilities, etc.

Change-Id: I8398728cd218f8c6e98f7a4a7f2f342391d865e4
2 files changed