Panic when one of the files to merge contains conflicts in diff2 format #520

thread 'main' panicked at src/line_based.rs:27:10:
diffy-imara returned a merge that we cannot parse the conflicts of: "Mergiraf cannot solve conflicts displayed in the diff2 style"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

If `mergiraf solve` is invoked on files where one of the revisions already contains conflicts represented in diff2 format (without the base part), then this leads to a panic: ``` thread 'main' panicked at src/line_based.rs:27:10: diffy-imara returned a merge that we cannot parse the conflicts of: "Mergiraf cannot solve conflicts displayed in the diff2 style" note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace ``` Even though such cases should only happen when the user has committed a file with conflicts into git (which is bad), we should make sure that: * we don't panic * any conflict that is already present in one of the base revisions (be it in diff2 / diff3 format) isn't treated as an actual conflict that mergiraf is meant to solve (as it's part of the source). This could be achieved by: * dynamically picking a conflict marker size which doesn't appear in the source revisions, or * plugging into diffy-imara at a lower level, so that we don't have to re-parse its output A reproducing test case is attached (taken from https://github.com/qiskit/qiskit-terra)