fix(cherry-pick): Abort conflicting cherry-pick before fallback

[ChristophWurst]

Fixes

10:09:48.548Z ERROR probot: Backport to stable25 failed
10:09:48.567Z ERROR probot:
  error: Cherry-picking is not possible because you have unmerged files.
  hint: Fix them up in the work tree, and then use 'git add/rm <file>'
  hint: as appropriate to mark resolution and make a commit.
  fatal: cherry-pick failed
  
  --
  Error: error: Cherry-picking is not possible because you have unmerged files.
  hint: Fix them up in the work tree, and then use 'git add/rm <file>'
  hint: as appropriate to mark resolution and make a commit.
  fatal: cherry-pick failed
  
      at toError (/app/node_modules/simple-git/promise.js:90:14)
      at /app/node_modules/simple-git/promise.js:61:36
      at Git.<anonymous> (/app/node_modules/simple-git/src/git.js:633:18)
      at Function.Git.fail (/app/node_modules/simple-git/src/git.js:1475:18)
      at fail (/app/node_modules/simple-git/src/git.js:1433:20)
      at /app/node_modules/simple-git/src/git.js:1442:16
      at runMicrotasks (<anonymous>)
      at processTicksAndRejections (internal/process/task_queues.js:93:5)

Leftover of #362