plumbing/object: avoid O(N^2) string building when decoding commit me…

.github/workflows/test.yml

@@ -0,0 +1,18 @@
+on: [push, pull_request]
+name: Test
+jobs:
+  test:
+    strategy:
+      matrix:
+        go-version: [1.12.x, 1.13.x, 1.14.x]
+        platform: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.platform }}
+    steps:
+    - name: Install Go
+      uses: actions/setup-go@v1
+      with:
+        go-version: ${{ matrix.go-version }}
+    - name: Checkout code
+      uses: actions/checkout@v2
+    - name: Test
+      run: go test -v ./...

plumbing/object/commit.go

@@ -177,6 +177,7 @@ func (c *Commit) Decode(o plumbing.EncodedObject) (err error) {
 
 	var message bool
 	var pgpsig bool
+	var msgbuf bytes.Buffer
 	for {
 		line, err := r.ReadBytes('\n')
 		if err != nil && err != io.EOF {
@@ -221,13 +222,15 @@ func (c *Commit) Decode(o plumbing.EncodedObject) (err error) {
 				pgpsig = true
 			}
 		} else {
-			c.Message += string(line)
+			msgbuf.Write(line)
 		}
 
 		if err == io.EOF {
-			return nil
+			break
 		}
 	}
+	c.Message = msgbuf.String()
+	return nil
 }
 
 // Encode transforms a Commit into a plumbing.EncodedObject.