merge: make non-conflicted debug string for Merge shorter

Resolves states are most common and the current format is pretty
verbose. Let's print it as if `Merge` were an enum with `Resolved` and
`Conflicted` variants instead.

lib/src/merge.rs

@@ -17,6 +17,7 @@
 
 use std::borrow::Borrow;
 use std::collections::HashMap;
+use std::fmt::{Debug, Formatter};
 use std::hash::Hash;
 use std::io::Write;
 use std::iter::zip;
@@ -104,12 +105,27 @@ where
 /// There is exactly one more `adds()` than `removes()`. When interpreted as a
 /// series of diffs, the merge's (i+1)-st add is matched with the i-th
 /// remove. The zeroth add is considered a diff from the non-existent state.
-#[derive(PartialEq, Eq, Hash, Clone, Debug)]
+#[derive(PartialEq, Eq, Hash, Clone)]
 pub struct Merge<T> {
     removes: Vec<T>,
     adds: Vec<T>,
 }
 
+impl<T: Debug> Debug for Merge<T> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), std::fmt::Error> {
+        // Format like an enum with two variants to make it less verbose in the common
+        // case of a resolved state.
+        if self.removes.is_empty() {
+            f.debug_tuple("Resolved").field(&self.adds[0]).finish()
+        } else {
+            f.debug_struct("Conflicted")
+                .field("removes", &self.removes)
+                .field("adds", &self.adds)
+                .finish()
+        }
+    }
+}
+
 impl<T> Merge<T> {
     /// Creates a new merge object from the given removes and adds.
     pub fn new(removes: Vec<T>, adds: Vec<T>) -> Self {

lib/tests/test_conflicts.rs

@@ -304,7 +304,7 @@ line 5 right
         @r###"
     Some(
         [
-            Merge {
+            Conflicted {
                 removes: [
                     "line 1\nline 2\n",
                 ],
@@ -313,13 +313,10 @@ line 5 right
                     "line 1 right\nline 2\n",
                 ],
             },
-            Merge {
+            Resolved(
-                removes: [],
-                adds: [
                 "line 3\n",
-                ],
+            ),
-            },
+            Conflicted {
-            Merge {
                 removes: [
                     "line 4\nline 5\n",
                 ],
@@ -464,13 +461,10 @@ line 5
         @r###"
     Some(
         [
-            Merge {
+            Resolved(
-                removes: [],
-                adds: [
                 "line 1\n",
-                ],
+            ),
-            },
+            Conflicted {
-            Merge {
                 removes: [
                     "line 2\nline 3\nline 4\n",
                 ],
@@ -479,12 +473,9 @@ line 5
                     "right\n",
                 ],
             },
-            Merge {
+            Resolved(
-                removes: [],
-                adds: [
                 "line 5\n",
-                ],
+            ),
-            },
         ],
     )
     "###
@@ -517,13 +508,10 @@ line 5
         @r###"
     Some(
         [
-            Merge {
+            Resolved(
-                removes: [],
-                adds: [
                 "line 1\n",
-                ],
+            ),
-            },
+            Conflicted {
-            Merge {
                 removes: [
                     "line 2\nline 3\nline 4\n",
                     "line 2\nline 3\nline 4\n",
@@ -534,12 +522,9 @@ line 5
                     "line 2\nforward\nline 3\nline 4\n",
                 ],
             },
-            Merge {
+            Resolved(
-                removes: [],
-                adds: [
                 "line 5\n",
-                ],
+            ),
-            },
         ],
     )
     "###