Add missing separator when merging comments

The same translatable strings with different comments results in merged comments without any separator.

Input Ruby file:
```ruby
# TRANSLATORS: this is a button label
_("foo")
# TRANSLATORS: this is help text description
_("foo")
```

Result:
```
#. TRANSLATORS: this is a button labelTRANSLATORS: this is help text description
#: ../test.rb:2 ../test.rb:4
msgid "foo"
```

After applying the fix:
```
#. TRANSLATORS: this is a button label
#. TRANSLATORS: this is help text description
#: ../test.rb:2 ../test.rb:4
msgid "foo"
```