support rebasing existing branch

dot_local/bin/executable_xxbranch

@@ -1,10 +1,27 @@
 #!/bin/sh
-if [ $# -eq 1 ]; then
-	branch=$1
-	git fetch upstream master
-	git checkout upstream/master
-	git checkout -b "$branch"
-else
-	echo "Please specify new branch."
+set -eu
+
+if [ $# -ne 1 ]; then
+  echo "Usage: $(basename "$0") <branch>" >&2
+  exit 2
 fi
 
+branch="$1"
+base_remote="upstream"
+base_branch="master"
+base_ref="$base_remote/$base_branch"
+
+# avoid rebasing with a dirty tree
+if ! git diff --quiet || ! git diff --cached --quiet; then
+  echo "Working tree has uncommitted changes; commit/stash first." >&2
+  exit 2
+fi
+
+git fetch "$base_remote" "$base_branch"
+
+if git show-ref --verify --quiet "refs/heads/$branch"; then
+  git checkout "$branch"
+  git rebase "$base_ref"
+else
+  git checkout -b "$branch" "$base_ref"
+fi