[lit] Make internal diff work in pipelines

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 file -
 # RUN: not diff file1 file2 | FileCheck %s
```

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` cannot
currently be used in pipelines and doesn't recognize `-` as a
command-line option.

To enable pipelines, this patch moves lit's `diff` implementation into
an out-of-process script, similar to lit's `cat` implementation.  A
follow-up patch will implement `-` to mean stdin.

Reviewed By: probinson, stella.stamenova

Differential Revision: https://reviews.llvm.org/D66574

llvm-svn: 374388

2 files changed, 15 insertions, 3 deletions

diff --git a/llvm/utils/lit/tests/Inputs/shtest-shell/diff-error-0.txt b/llvm/utils/lit/tests/Inputs/shtest-shell/diff-error-0.txt
deleted file mode 100644
index 81888cf8197..00000000000
--- a/llvm/utils/lit/tests/Inputs/shtest-shell/diff-error-0.txt
+++ /dev/null
@@ -1,3 +0,0 @@
-# Check error on a unsupported diff (cannot be part of a pipeline).
-#
-# RUN: diff diff-error-0.txt diff-error-0.txt | echo Output
diff --git a/llvm/utils/lit/tests/Inputs/shtest-shell/diff-pipes.txt b/llvm/utils/lit/tests/Inputs/shtest-shell/diff-pipes.txt
new file mode 100644
index 00000000000..ce0abca1661
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/shtest-shell/diff-pipes.txt
@@ -0,0 +1,15 @@
+# RUN: echo foo > %t.foo
+# RUN: echo bar > %t.bar
+
+# Check output pipe.
+# RUN: diff %t.foo %t.foo | FileCheck -allow-empty -check-prefix=EMPTY %s
+# RUN: diff -u %t.foo %t.bar | FileCheck %s && false || true
+
+# Fail so lit will print output.
+# RUN: false
+
+# CHECK:      @@
+# CHECK-NEXT: -foo
+# CHECK-NEXT: +bar
+
+# EMPTY-NOT: {{.}}