[Development] [IMPORTANT] your commits in stable branch perhaps lost after dev branch merged

[Qi Liang]

> Merging is not an easy task. (I know it, I was doing the merge during Qt 4.x
> times)
> It is easy for someone unexperimented to mess up.
> Which is why I think only experimented developer should do the merge.

Not that easy like Qt 4.x ages(we only have 1 repo, and we could do that merge per week or per month, or just before a release), now we have about 10 repos, and after open governance, changes are more. And sometimes, a merge depends on another merge(just like qtdeclarative depends on qtbase). stable->dev merges happened like daily.

> A merge is between two branches.   And the merge operation is commutative.
> Think about it like:
>    merge(stable , dev) == merge (dev, stable)
>
> // usually
> tmp = merge(stable, dev);
> dev = tmp;
> 
> // this time
> tmp = merge(stable, dev);
> stable = tmp;

// usually or before
stable = stable; // 4.x-1
dev = merge(stable, dev); // 4.x, stable->dev

// this time
stable = merge(dev, stable); // dev->stable
dev = stable;
// then here you can see stable doesn't have much meaning when release cycle is short enough.

Regards,
Liang