Branch Mainline :

commit A [introduced after I branched to dev from mainline]
commit B
commit C
.... so on

Branch  Dev:

Commit P  [not pushed to remote]
Commit Q  [not pushed to remote]
more than 10 different commits in between [pushed to remote dev branch]
Commit B [Based on mainline]

Required result in Branch Mainline:

Commit allDev [A single commit with everything from the dev branch after Commit B ]
Commit A [This might need a merge commit since overlapping code from dev]
Commit B
Commit C
... so on