Some tests require `ed` and fail if it is not installed

[cakebaker]

When running cargo test the following tests fail if ed is not installed:

• ed_diff::tests::test_permutations
• ed_diff::tests::test_permutations_empty_lines
• ed_diff::tests::test_permutations_reverse

They fail because they call Command::new("ed").

[oSoMoN]

The Git for Windows installer installs a number of standard Unix executables in C:\Program Files\Git\usr\bin\. Unfortunately, ed isn't one of them.