Git rebase, skip merge-commits

Looking at your original workflow, it seems like you want to ignore merges, the only problem is using -i for an interactive rebase, which preserves merges.

git checkout -b temp  
git rebase --onto master hack temp
   * Big drawback is gone!
git checkout master  
git merge temp  
git branch -d temp

Should work exactly how you want. This probably doesn't exactly solve your "general case," though.

Simple case

If the state of your repo is

  hack---F1----M1----F2 [feature]
 /            /
C1-----C2----C3 [master]

and you want to arrive at

  hack---F1----M1----F2 [feature]
 /            /
C1-----C2----C3----F1'----F2' [HEAD=master]

you should use git cherry-pick, not git rebase -i (no need to juggle with interactive rebase, here):

git checkout master
git cherry-pick <commit-ID-of-F1> <commit-ID-of-F2>

General case

Correct me if I'm wrong, but I understand what you mean by general case as

cherry-pick, on top of master, all the non-merge commits between hack (exclusive) and the tip of feature (inclusive).

In the following, I'm assuming that is indeed what you mean.

As you've rightfully noted in your comment, the approach outlined above doesn't scale very gracefully as the number of commits to manually cherry-pick increases:

  hack---F1---F2--- .... --- F68--M1---F67---...---F99 [feature]
 /                               /
C1-------------C2---------------C3 [master]

However, you can get git rev-list to automatically generate the list of revisions of interest, using

git rev-list --reverse --no-merges --first-parent <commit-ID-of-hack>..feature

Edit: you also need the --first-parent flag to avoid collecting commits such as C1 and C2, and --reverse flag, so that commits get cherry-picked in the desired order.

You can pass the output of that command to git cherry-pick:

git checkout master
git cherry-pick `git rev-list --reverse --no-merges --first-parent <commit-ID-of-hack>..feature`

which would yield

  hack---F1---F2--- .... --- F68--M1---F67---...---F99 [feature]
 /                               /
C1-------------C2---------------C3---F1'---F2'---...---F99' [HEAD=master]