Fix panic with custom Debug impl returning an empty string

The panic was 'attempt to subtract with overflow' because
the first difference is addition.

PrettyString is from idubrov:
#24 (comment)

src/format_changeset.rs

@@ -36,7 +36,8 @@ pub fn format_changeset(f: &mut fmt::Formatter, changeset: &Changeset) -> fmt::R
                 }
             }
             Difference::Add(ref added) => {
-                match diffs.get(i - 1) {
+                let prev = i.checked_sub(1).and_then(|x| diffs.get(x));
+                match prev {
                     Some(&Difference::Rem(ref removed)) => {
                         // The addition is preceded by an removal.
                         //
@@ -52,7 +53,8 @@ pub fn format_changeset(f: &mut fmt::Formatter, changeset: &Changeset) -> fmt::R
                 };
             }
             Difference::Rem(ref removed) => {
-                match diffs.get(i + 1) {
+                let next = i.checked_add(1).and_then(|x| diffs.get(x));
+                match next {
                     Some(&Difference::Add(_)) => {
                         // The removal is followed by an addition.
                         //

tests/pretty_string.rs

@@ -0,0 +1,28 @@
+#[macro_use]
+extern crate pretty_assertions;
+
+use std::fmt;
+/// Wrapper around string slice that makes debug output `{:?}` to print string same way as `{}`.
+/// Used in different `assert*!` macros in combination with `pretty_assertions` crate to make
+/// test failures to show nice diffs.
+#[derive(PartialEq, Eq)]
+#[doc(hidden)]
+pub struct PrettyString<'a>(pub &'a str);
+
+/// Make diff to display string as multi-line string
+impl<'a> fmt::Debug for PrettyString<'a> {
+  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    f.write_str(self.0)
+  }
+}
+
+#[test]
+#[should_panic(expected = r#"assertion failed: `(left == right)`
+
+Diff < left / right > :
+>foo
+
+"#)]
+fn assert_eq_empty_first() {
+    assert_eq!(PrettyString(""), PrettyString("foo"));
+}