Refactor create_gitea_pr() in common_test_utils.py to leverage the
'new_branch' parameter in the Gitea 'diffpatch' API call. This allows
for automatic creation of the target branch while applying the diff,
eliminating the need for explicit branch creation using the branches
endpoint. This also fixed strange behavior when diffpatch damaged
history of precreated branch
We can reset current worktree and clobber it with the merged changes.
We want to emulate `git merge -s theirs` strategy while
`git merge -Xtheirs` only picks `theirs` in case of conflicts.
So, resetting the changes and reading exact is sufficient
`git read-tree -u` updates the current work tree so we do not have
unstaged changes.
