update: shallow fetch for git_init_if_necessary

Users shouldn't need to fetch all of git history.

Closes Homebrew/homebrew#49903.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/cmd/update.sh

@@ -42,22 +42,21 @@ which_git() {
 }
 
 git_init_if_necessary() {
-  set -e
-  trap '{ rm -rf .git; exit 1; }' EXIT
-
   if [[ ! -d ".git" ]]
   then
+    set -e
+    trap '{ rm -rf .git; exit 1; }' EXIT
     git init
     git config --bool core.autocrlf false
     git config remote.origin.url https://github.com/Homebrew/homebrew.git
     git config remote.origin.fetch "+refs/heads/*:refs/remotes/origin/*"
-    git fetch origin
+    git fetch --force --depth=1 origin refs/heads/master:refs/remotes/origin/master
     git reset --hard origin/master
     SKIP_FETCH_HOMEBREW_REPOSITORY=1
-  fi
-
     set +e
     trap - EXIT
+    return
+  fi
 
   if [[ "$(git remote show origin -n)" = *"mxcl/homebrew"* ]]
   then