update-bash: fix git_init_if_necessary

* Make sure `.git` directory be deleted at any error. So we won't have a
  stale setup.
* Run `git fetch` and `git reset` when initialize git in the first time.
  Otherwise, we will get error and merging problem afterwards.

Closes Homebrew/homebrew#48509.

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

Library/Homebrew/cmd/update-bash.sh

@@ -21,17 +21,26 @@ which_git() {
 }
 
 git_init_if_necessary() {
+  set -e
+  trap '{ rm -rf .git; exit 1; }' EXIT
+
   if [[ ! -d ".git" ]]
   then
     git init -q
     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 reset --hard origin/master
+    SKIP_FETCH_HOMEBREW_REPOSITORY=1
   fi
 
+  set +e
+  trap - EXIT
+
   if [[ "$(git remote show origin -n)" = *"mxcl/homebrew"* ]]
   then
-    git remote set-url origin https://github.com/Homebrew/homebrew.git
+    git remote set-url origin https://github.com/Homebrew/homebrew.git &&
     git remote set-url --delete origin ".*mxcl\/homebrew.*"
   fi
 }
@@ -295,6 +304,7 @@ EOS
   for DIR in "$HOMEBREW_REPOSITORY" "$HOMEBREW_LIBRARY"/Taps/*/*
   do
     [[ -d "$DIR/.git" ]] || continue
+    [[ -n "$SKIP_FETCH_HOMEBREW_REPOSITORY" && "$DIR" = "$HOMEBREW_REPOSITORY" ]] && continue
     cd "$DIR" || continue
     UPSTREAM_BRANCH="$(upstream_branch)"
     # the refspec ensures that the default upstream branch gets updated