When using lit's internal shell, RUN lines like the following
accidentally execute an external `diff` instead of lit's internal
`diff`:
```
# RUN: program | diff -U1 file -
```
Such cases exist now, in `clang/test/Analysis` for example. We are
preparing patches to ensure lit's internal `diff` is called in such
cases, which will then fail because lit's internal `diff` doesn't
recognize `-U` as a command-line option. This patch adds `-U`
support.
Reviewed By: rnk
Differential Revision: https://reviews.llvm.org/D68668
llvm-svn: 374814
7e385bd2f5