Fix fatal: corrupt patch error in unified diff format

plumbing/format/diff/unified_encoder.go

@@ -237,9 +237,13 @@ func (c *hunksGenerator) addLineNumbers(la, lb int, linesBefore int, i int, op O
 	// we need to search for a reference for the next diff
 	switch {
 	case linesBefore != 0 && c.ctxLines != 0:
-		clb = lb - c.ctxLines + 1
+		if lb > c.ctxLines {
+			clb = lb - c.ctxLines + 1
+		} else {
+			clb = 1
+		}
 	case c.ctxLines == 0:
-		clb = lb - c.ctxLines
+		clb = lb
 	case i != len(c.chunks)-1:
 		next := c.chunks[i+1]
 		if next.Type() == op || next.Type() == Equal {

plumbing/format/diff/unified_encoder_test.go

@@ -150,6 +150,43 @@ var oneChunkPatchInverted Patch = testPatch{
 }
 
 var fixtures []*fixture = []*fixture{{
+	patch: testPatch{
+		message: "",
+		filePatches: []testFilePatch{{
+			from: &testFile{
+				mode: filemode.Regular,
+				path: "README.md",
+				seed: "hello\nworld\n",
+			},
+			to: &testFile{
+				mode: filemode.Regular,
+				path: "README.md",
+				seed: "hello\nbug\n",
+			},
+			chunks: []testChunk{{
+				content: "hello",
+				op:      Equal,
+			}, {
+				content: "world",
+				op:      Delete,
+			}, {
+				content: "bug",
+				op:      Add,
+			}},
+		}},
+	},
+	desc:    "positive negative number",
+	context: 2,
+	diff: `diff --git a/README.md b/README.md
+index 94954abda49de8615a048f8d2e64b5de848e27a1..f3dad9514629b9ff9136283ae331ad1fc95748a8 100644
+--- a/README.md
++++ b/README.md
+@@ -1,2 +1,2 @@
+ hello
+-world
++bug
+`,
+}, {
 	patch: testPatch{
 		message: "",
 		filePatches: []testFilePatch{{