[loganlinn]

  sync = "!f() { echo Syncing $1 with master && git checkout master && git pull && git checkout $1 && git rebase master; }; f"

I believe this is equivalent to (from topic branch)

  sync = "git pull --rebase origin master"

[etherealG]

even better, "git pull --rebase" on it's own. the origin and master can both be custom per branch and work as defaults if not specified.