Try to fix Encoding::CompatibilityError

```
Encoding::CompatibilityError: invalid byte sequence in US-ASCII
/home/git/git.ruby-lang.org/bin/commit-email.rb:124:in `strip'
/home/git/git.ruby-lang.org/bin/commit-email.rb:124:in `git_show'
/home/git/git.ruby-lang.org/bin/commit-email.rb:31:in `build'
/home/git/git.ruby-lang.org/bin/commit-email.rb:185:in `block (2 levels) in main'
/home/git/git.ruby-lang.org/bin/commit-email.rb:184:in `map'
/home/git/git.ruby-lang.org/bin/commit-email.rb:184:in `block in main'
/home/git/git.ruby-lang.org/bin/commit-email.rb:182:in `each'
/home/git/git.ruby-lang.org/bin/commit-email.rb:182:in `each_slice'
/home/git/git.ruby-lang.org/bin/commit-email.rb:182:in `each'
/home/git/git.ruby-lang.org/bin/commit-email.rb:182:in `flat_map'
/home/git/git.ruby-lang.org/bin/commit-email.rb:182:in `main'
/home/git/git.ruby-lang.org/bin/commit-email.rb:382:in `<main>'
```

Author: hsbt

bin/commit-email.rb

@@ -126,7 +126,7 @@ def git_show(revision, format:)
 
   def git(*args)
     command = ['git', '-C', @repo_path, *args]
-    output = with_gitenv { IO.popen(command, &:read) }
+    output = with_gitenv { IO.popen(command, external_encoding: 'UTF-8', &:read) }
     unless $?.success?
       raise GitCommandFailure, "failed to execute '#{command.join(' ')}':\n#{output}"
     end