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

[phil-blain]

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.