Resolving Conflicts in Git

First add the upstream remote then fetch it. 1

git remote add upstream https://repoA
git fetch upstream

Check out the branch (in this case, notification) and auto-merge it with the upstream master. 1

git checkout notification
git merge upstream/master

Resolve the conflicts by fully accept my or their version2:

Accept my version (local, ours):

git checkout --ours -- [filename]
git add [filename]
git commit -m "merged bla bla"

Accept their version (remote, theirs):

git checkout --theirs -- [filename]
git add [filename]
git commit -m "merged bla bla"

Do all conflict files with either mine or theirs:

git merge --strategy-option ours

or

git merge --strategy-option theirs