Q:

Merge large git commit by splitting it into small commits on master branch

I have a branch in git which would apply cleanly to master. It contains one commit with 10 features which would apply cleanly to master. But i would like to merge the changes into master features based. The features changes are not on file level (so one change file may contain changes for 2-3 features, but they can be split up easily fortunatly).  But any merge strategy  on whole file content fails therefore, it would be more like merge  lines 10-20 from file 1 and lines 70-80 from file and lines 100 from file 3 for one feature.   Is there any kind  of editor which would 

allow me to pick the lines in the huge commit
allow me to merge the picked lines into a small commit into master

--> in the end it would be "one large commit on a branch"  converted into 10 commits on master, feature by feature
Is there any tool (chain) which would support such a workflow visually? 

A:

Given that the work you'd like to merge piecemeal into master is in one commit on a non-master all-in-one branch, I would recommend breaking up the all-in-one commit first, then merging one smaller commit at a time into master. That way, as opposed to merging in a patch, your Git history will be able to track where the changes came from.
There are multiple ways to accomplish this. One way would be to create a second piecemeal branch off of master from the same start point as your all-in-one branch, merge all-in-one into the piecemeal branch without committing, unstage any staged changes, then start breaking up the unstaged work into smaller commits on piecemeal.
To break unstaged work into multiple commits with visual assistance, git add --patch . allows you to stage (or not) each current unstaged 'hunk' of code to the Git index in the console.
git gui provides a graphical way to do the same: select unstaged file, right click section of file, click Stage Hunk For Commit. See http://www.adamfranco.com/2009/01/13/git-tip-of-the-day-stage-hunks/ for stage by stage pictures and further details.

