[FIX] Diff: Support Python < 2.7.2

The function named '_format_range_unified' was introduced in 2.7.2:
https://hg.python.org/cpython/file/8527427914a2/Lib/difflib.py#l1147
This adds a backported version of so that Python 2.6, 2.7.0 and 2.7.1
are supported

It also fixes a problem with non-indexed placeholders for format which
aren't supported in Python 2.6.

Bug: T76276
Change-Id: I2f2799f434d3a0782999f94a307c9a081a004014
Merge "[FIX] Diff: Support Python < 2.7.2"