Just specify your local branch1 or remote origin/branch2
git diff branch1 origin/branch2 #show diff referenced from second branch
git diff branch1 branch2 file.txt #show diff only between file.txt
Just store diff into a patch file, edit patch if needed and apply patch:
git diff commitid1 commitid2 > file.patch #edit diff to merge in file
git apply file.patch #Apply diff of patch file into current branch
