fix(internal/gapicgen): don't regen files that have been deleted (#3471)

 Currently this method will find all the files that haven been
 updated since the last hash. This is then used to find out
 which packages will need to be regenerated. We should not try to
 regenerate files that have been deleted, this causes a file does
 not exist error when running the generator.

 The provided flag restricts to files that:
   - (A) Added
   - (C) Copied
   - (M) Modified
   - (R) Renamed

Author: codyoss

internal/gapicgen/generator/git.go

@@ -134,7 +134,12 @@ func CommitsSinceHash(gitDir, hash string, inclusive bool) ([]string, error) {
 // hash for the given gitDir.
 func UpdateFilesSinceHash(gitDir, hash string) ([]string, error) {
 	out := bytes.NewBuffer(nil)
-	c := command("git", "diff-tree", "--no-commit-id", "--name-only", "-r", fmt.Sprintf("%s..HEAD", hash))
+	// The provided diff-filter flags restricts to files that have been:
+	// - (A) Added
+	// - (C) Copied
+	// - (M) Modified
+	// - (R) Renamed
+	c := command("git", "diff-tree", "--no-commit-id", "--name-only", "--diff-filter=ACMR", "-r", fmt.Sprintf("%s..HEAD", hash))
 	c.Stdout = out
 	c.Dir = gitDir
 	if err := c.Run(); err != nil {