Using `git resolve-conflict` with "both added" conflict shows a fatal error

For "both added" conflicts, `git show :1:path/to/file` yields a fatal error since there is nothing in stage 1. This does not crash the script since the script does not use `set -e`, and the file where the output from  `git show`  is redirected ends up being empty (which is the right result), but it would be nice to detect that situation and avoid the command in that case.