Do not override existing versions when batching, unless forced

[miniscruff]

Discussed in #405, creating a version using batch should not replace an existing file unless you use force.

Reproduction Steps

1. Add some change fragments with new command
2. Generate a version with batch command (batch v1.3.0)
3. Add more change fragments with new
4. Generate the same version again (batch v1.3.0) should not work
5. Generating the same version with force (batch v1.3.0 --force) should work

What happened

Version file is overridden with new contents.

Expected behavior

Changie should error out when batching the same version again.
This should be allowed if you add the new option of --force.