Fix patch file creation using Gazelle in diff mode (#1915)

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**What type of PR is this?**
  Bug fix


**What package or component does this PR mostly affect?**
  cmd/gazelle

**What does this PR do? Why is it needed?**
This PR adds a workaround to strip trailing newlines from the resulting
diff when running Gazelle in `mode=diff`. This aligns with the expected
behavior of modern diff tools, which will ignore the last newline in a
file. This is implemented to resolve an issue where modifying the end of
a file caused Gazelle to output a diff that was invalid, due to the
newlines. Applying patches generated in this case could not be applied
when using `git apply` and would apply, but with a warning identifying
this issues when using the system `patch` utility.

**Which issues(s) does this PR fix?**

Fixes #1916 

**Other notes for review**
How to Reproduce:
Edit a `BUILD.bazel` file in this repository, and then run `bazel run
//:gazelle -- -mode=diff -patch=temp.patch`. Next, run `git apply -p0
temp.patch`. Without these changes, patch application will fail, due to
the issue described above.

cmd/gazelle/diff.go

@@ -74,6 +74,15 @@ func diffFile(c *config.Config, f *rule.File) error {
 	} else {
 		diff.ToFile = outPath
 	}
+	// Trim off trailing newline from the end of the file.  Modern diff tools typically
+	// handle this, however difflib does not and is no longer maintained, so we need to
+	// handle it here.
+	if len(diff.A) > 0 {
+		if diff.A[len(diff.A)-1] == "\n" {diff.A = diff.A[:len(diff.A)-1]}
+	}
+	if len(diff.B) > 0 {
+		if diff.B[len(diff.B)-1] == "\n" {diff.B = diff.B[:len(diff.B)-1]}
+	}
 
 	uc := getUpdateConfig(c)
 	var out io.Writer = os.Stdout

cmd/gazelle/diff_test.go

@@ -50,18 +50,17 @@ func TestDiffExisting(t *testing.T) {
 		Content: `
 --- BUILD.bazel	1970-01-01 00:00:00.000000001 +0000
 +++ BUILD.bazel	1970-01-01 00:00:00.000000001 +0000
-@@ -1,3 +1,11 @@
+@@ -1,2 +1,10 @@
 +load("@io_bazel_rules_go//go:def.bzl", "go_library")
  
  # gazelle:prefix example.com/hello
- 
++
 +go_library(
 +    name = "hello",
 +    srcs = ["hello.go"],
 +    importpath = "example.com/hello",
 +    visibility = ["//visibility:public"],
 +)
-+
 `,
 	})
 	testtools.CheckFiles(t, dir, want)
@@ -88,7 +87,7 @@ func TestDiffNew(t *testing.T) {
 		Content: `
 --- /dev/null	1970-01-01 00:00:00.000000001 +0000
 +++ BUILD.bazel	1970-01-01 00:00:00.000000001 +0000
-@@ -0,0 +1,9 @@
+@@ -0,0 +1,8 @@
 +load("@io_bazel_rules_go//go:def.bzl", "go_library")
 +
 +go_library(
@@ -97,7 +96,6 @@ func TestDiffNew(t *testing.T) {
 +    importpath = "example.com/hello",
 +    visibility = ["//visibility:public"],
 +)
-+
 `,
 	})
 	testtools.CheckFiles(t, dir, want)
@@ -160,7 +158,7 @@ func TestDiffReadWriteDir(t *testing.T) {
 	wantPatch := fmt.Sprintf(`
 --- %s	1970-01-01 00:00:00.000000001 +0000
 +++ %s	1970-01-01 00:00:00.000000001 +0000
-@@ -1 +1,11 @@
+@@ -1 +1,10 @@
 +load("@io_bazel_rules_go//go:def.bzl", "go_library")
 +
  # gazelle:prefix example.com/hello
@@ -171,7 +169,6 @@ func TestDiffReadWriteDir(t *testing.T) {
 +    importpath = "example.com/hello",
 +    visibility = ["//visibility:public"],
 +)
-+
 `,
 		filepath.Join(dir, "read", "BUILD.bazel"),
 		filepath.Join(dir, "write", "BUILD.bazel"))

tests/diff_mode/expectedStdout.txt

@@ -1,6 +1,6 @@
 --- /dev/null	1970-01-01 00:00:00.000000001 +0000
 +++ BUILD.bazel	1970-01-01 00:00:00.000000001 +0000
-@@ -0,0 +1,10 @@
+@@ -0,0 +1,9 @@
 +filegroup(
 +    name = "all_files",
 +    testonly = True,
@@ -9,5 +9,4 @@
 +        "WORKSPACE",
 +    ],
 +    visibility = ["//visibility:public"],
-+)
-+
++)

tests/loads_from_flag/expectedStdout.txt

@@ -1,7 +1,6 @@
 --- /dev/null	1970-01-01 00:00:00.000000001 +0000
 +++ BUILD.bazel	1970-01-01 00:00:00.000000001 +0000
-@@ -0,0 +1,4 @@
+@@ -0,0 +1,3 @@
 +load("@some//repo:rules.bzl", "custom_rule")
 +
-+custom_rule(name = "gen")
-+
++custom_rule(name = "gen")