Truncate rebase conflict status commit list if too long

[greened]

When I run magit over TRAMP, do a very large rebase (such as rebasing downstream changes on top of a new upstream release) and encounter a conflict, magit spends a lot of time inserting buffers for each commit in the rebase sequence to display the status. Since this is over TRAMP, each such insert requires network operations and it proceeds very slowly, to the point where I have to quit and lose the ability to operate on the rebase from within magit.

It would be helpful to have an option to limit the number of such commits magit tries to display in the rebase status. I really only care about the ~10 commits surrounding the current conflicting commit. I don't care about things 1000 commits in the future. I think a simple customization option would be fine. By default it would be "infinite" to maintain current behavior but could be set to some integer to limit the work magit does on each conflict.

[greened]

In addition, it would be nice if a quit during the "insert a buffer for each rebase commit" sequence would simply stop that activity but allow magit to continue rendering the other parts of the status (the file state/conflict diff) so that we can use normal magit operations to visit the files with conflicting changes.