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