Revert "update: checkout branch unconditionally"

This reverts commit 0525c9eeea27eff1a6de05e7a01ec268ae07437b.
Which breaks `brew update --rebase`.

1 files changed, 9 insertions, 6 deletions

diff --git a/Library/Homebrew/cmd/update.sh b/Library/Homebrew/cmd/update.sh
index 0c2faf448..07847a0ba 100644
--- a/Library/Homebrew/cmd/update.sh
+++ b/Library/Homebrew/cmd/update.sh
@@ -211,13 +211,16 @@ pull() {
     STASHED="1"
   fi
 
-  # Recreate and check out `#{upstream_branch}` if unable to fast-forward
-  # it to `origin/#{@upstream_branch}`. Otherwise, just check it out.
-  if git merge-base --is-ancestor "$UPSTREAM_BRANCH" "origin/$UPSTREAM_BRANCH" &>/dev/null
+  if [[ "$INITIAL_BRANCH" != "$UPSTREAM_BRANCH" && -n "$INITIAL_BRANCH" ]]
   then
-    git checkout --force "$UPSTREAM_BRANCH" "${QUIET_ARGS[@]}"
-  else
-    git checkout --force -B "$UPSTREAM_BRANCH" "origin/$UPSTREAM_BRANCH" "${QUIET_ARGS[@]}"
+    # Recreate and check out `#{upstream_branch}` if unable to fast-forward
+    # it to `origin/#{@upstream_branch}`. Otherwise, just check it out.
+    if git merge-base --is-ancestor "$UPSTREAM_BRANCH" "origin/$UPSTREAM_BRANCH" &>/dev/null
+    then
+      git checkout --force "$UPSTREAM_BRANCH" "${QUIET_ARGS[@]}"
+    else
+      git checkout --force -B "$UPSTREAM_BRANCH" "origin/$UPSTREAM_BRANCH" "${QUIET_ARGS[@]}"
+    fi
   fi
 
   INITIAL_REVISION="$(read_current_revision)"