Make the output of UnifiedDiffFormatter match diff -u

Make the output of UnifiedDiffFormatter match diff -u

GNU 'diff -u' prefixes each line with either a space, a '+', or a '-'.
UnifiedDiffFormatter does the same, but it also adds an additional column of
whitespace between the prefix and the line. GNU diff only does that in
non-unified mode.

Fix this by implementing lines() in UnifiedDiffFormatter, overriding the parent
class implementation.

Bug: T100069
Change-Id: I1bf1b8e6d1d5aceb2c3836548f492f7edebe5a12