Adding the diff for a single file to the DiffOperations interface. Also
renamed the existing methods for getting the list of modified files to
"listModifiedFiles" for clarity. The code for computing the base commit
for diffs against parent or auto-merge is also extracted because it's
used by the listModifiedFiles and the getModifiedFile methods.
Change-Id: I23f616d1c89f7ddad8e8fd0f526394475099d157
b34ed026f2