Answers for "git compare two branches"

git show diff between branches

git diff branch_1..branch_2

diff specific file git different branches

git diff mybranch master -- myfile.cs

git compare two branches

git diff branch1..branch2

git merge diff between branches

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