Add test for rename support in multi file diff.

[dmitshur]

Followup for #9 (comment).

This test passes now, but would fail before #9 was merged.

[dmitshur]

The sample_multi_file_rename.diff test case was created from an actual modified git commit, using git version 2.5.4 (Apple Git-61), rendered with the following flags:

git show -M HEAD

Before #9 was merged, the tests fails with:

go-diff $ go test -v ./...
?       sourcegraph.com/sourcegraph/go-diff/cmd/go-diff [no test files]
=== RUN   TestParseHunkNoChunksize
--- PASS: TestParseHunkNoChunksize (0.00s)
=== RUN   TestParseHunksAndPrintHunks
--- PASS: TestParseHunksAndPrintHunks (0.00s)
=== RUN   TestParseFileDiffAndPrintFileDiff
--- PASS: TestParseFileDiffAndPrintFileDiff (0.00s)
=== RUN   TestParseMultiFileDiffAndPrintMultiFileDiff
--- FAIL: TestParseMultiFileDiffAndPrintMultiFileDiff (0.00s)
    diff_test.go:159: sample_multi_file_rename.diff: got 2 instances of diff.FileDiff, expected 3
=== RUN   TestNoNewlineAtEnd
--- PASS: TestNoNewlineAtEnd (0.00s)
=== RUN   TestFileDiff_Stat
--- PASS: TestFileDiff_Stat (0.00s)
FAIL
FAIL    sourcegraph.com/sourcegraph/go-diff/diff    0.008s

After #9 is merged, all these tests pass.

[emidoots]

LGTM