Replace not supported option pull.rebase=preserve with pull.rebase=merges
preserve option is not supported anymore https://github.com/git/git/commit/52f1e82178e75bdf876799770922bc34fdf6fae4
merges option is supported since GIT 2.18.0 https://github.com/git/git/commit/1131ec98189e993dcc48043c4f8e8b0128f52055
Signed-off-by: Jiří Engelthaler EngyCZ@gmail.com