diff: Allow srcPrefix and dstPrefix to be configured

The default behavior here remains the same, but this change does
allow consumers of the UnifiedEncoder to set their own path prefixes
which will override the defaults of a/ and b/.

plumbing/format/diff/unified_encoder.go

@@ -38,6 +38,10 @@ type UnifiedEncoder struct {
 	// a change.
 	contextLines int
 
+	// srcPrefix and dstPrefix are prepended to file paths when encoding a diff.
+	srcPrefix string
+	dstPrefix string
+
 	// colorConfig is the color configuration. The default is no color.
 	color ColorConfig
 }
@@ -46,6 +50,8 @@ type UnifiedEncoder struct {
 func NewUnifiedEncoder(w io.Writer, contextLines int) *UnifiedEncoder {
 	return &UnifiedEncoder{
 		Writer:       w,
+		srcPrefix:    "a/",
+		dstPrefix:    "b/",
 		contextLines: contextLines,
 	}
 }
@@ -56,6 +62,18 @@ func (e *UnifiedEncoder) SetColor(colorConfig ColorConfig) *UnifiedEncoder {
 	return e
 }
 
+// SetSrcPrefix sets e's srcPrefix and returns e.
+func (e *UnifiedEncoder) SetSrcPrefix(prefix string) *UnifiedEncoder {
+	e.srcPrefix = prefix
+	return e
+}
+
+// SetDstPrefix sets e's dstPrefix and returns e.
+func (e *UnifiedEncoder) SetDstPrefix(prefix string) *UnifiedEncoder {
+	e.dstPrefix = prefix
+	return e
+}
+
 // Encode encodes patch.
 func (e *UnifiedEncoder) Encode(patch Patch) error {
 	sb := &strings.Builder{}
@@ -91,7 +109,8 @@ func (e *UnifiedEncoder) writeFilePatchHeader(sb *strings.Builder, filePatch Fil
 	case from != nil && to != nil:
 		hashEquals := from.Hash() == to.Hash()
 		lines = append(lines,
-			fmt.Sprintf("diff --git a/%s b/%s", from.Path(), to.Path()),
+			fmt.Sprintf("diff --git %s%s %s%s",
+				e.srcPrefix, from.Path(), e.dstPrefix, to.Path()),
 		)
 		if from.Mode() != to.Mode() {
 			lines = append(lines,
@@ -115,22 +134,22 @@ func (e *UnifiedEncoder) writeFilePatchHeader(sb *strings.Builder, filePatch Fil
 			)
 		}
 		if !hashEquals {
-			lines = e.appendPathLines(lines, "a/"+from.Path(), "b/"+to.Path(), isBinary)
+			lines = e.appendPathLines(lines, e.srcPrefix+from.Path(), e.dstPrefix+to.Path(), isBinary)
 		}
 	case from == nil:
 		lines = append(lines,
-			fmt.Sprintf("diff --git a/%s b/%s", to.Path(), to.Path()),
+			fmt.Sprintf("diff --git %s %s", e.srcPrefix+to.Path(), e.dstPrefix+to.Path()),
 			fmt.Sprintf("new file mode %o", to.Mode()),
 			fmt.Sprintf("index %s..%s", plumbing.ZeroHash, to.Hash()),
 		)
-		lines = e.appendPathLines(lines, "/dev/null", "b/"+to.Path(), isBinary)
+		lines = e.appendPathLines(lines, "/dev/null", e.dstPrefix+to.Path(), isBinary)
 	case to == nil:
 		lines = append(lines,
-			fmt.Sprintf("diff --git a/%s b/%s", from.Path(), from.Path()),
+			fmt.Sprintf("diff --git %s %s", e.srcPrefix+from.Path(), e.dstPrefix+from.Path()),
 			fmt.Sprintf("deleted file mode %o", from.Mode()),
 			fmt.Sprintf("index %s..%s", from.Hash(), plumbing.ZeroHash),
 		)
-		lines = e.appendPathLines(lines, "a/"+from.Path(), "/dev/null", isBinary)
+		lines = e.appendPathLines(lines, e.srcPrefix+from.Path(), "/dev/null", isBinary)
 	}
 
 	sb.WriteString(e.color[Meta])