Add sync

[geoffjay]

Adds a git-sync command to perform the equivalent of:

x-repo = !"f() { git config --get remote.origin.url | sed 's/^.*\\:\\(.*\\)\\.git/\\1/'; }; f"
x-fetch-all = fetch --no-tags --all -p
x-fetch-branch = !"f() { git fetch origin $1:$1; }; f"
x-merge-ff = merge --ff-only
x-branch-current = rev-parse --abbrev-ref HEAD
x-branch-default = !"f() { git remote show origin | grep HEAD | awk '{print $3}'; }; f"
x-branch-tidy = "!f() { git branch -vv | grep ': gone]' | awk '{print $1}' | xargs -n 1 git branch -D; }; f"
up = \
!"f() { \
  current=$(git x-branch-current); \
  default=$(git x-branch-default); \
  git x-fetch-all && \
  (git x-merge-ff || true) && \
  git x-branch-tidy && \
  if [ \"$current\" != \"$default\" ]; then git x-fetch-branch $default; fi; \
}; f"