Add start point to worktree add subcommand

worktree/_worktree

@@ -9,6 +9,14 @@ __worktree_branches() {
   _describe 'branch' branches
 }
 
+__worktree_start_points() {
+  local -a refs
+  refs=(
+    ${(fo)"$(git for-each-ref --format='%(refname:short)' refs/heads refs/remotes 2>/dev/null)"}
+  )
+  _describe 'start point' refs
+}
+
 __worktree_active_branches() {
   local -a branches
   branches=(${(fo)"$(git worktree list --porcelain 2>/dev/null | \
@@ -49,7 +57,9 @@ _worktree() {
       curcontext="${curcontext%:*:*}:worktree-cmd-$words[1]:"
       case $line[1] in
         (add)
-          _arguments '1:: :__worktree_branches'
+          _arguments \
+            '1:: :__worktree_branches' \
+            '2:: :__worktree_start_points'
           ;;
         (cd)
           _arguments '1:: :__worktree_all_branches'

worktree/worktree.plugin.zsh

@@ -81,14 +81,17 @@ __worktree_find_in() {
 
 worktree() {
   if [[ "$1" == "" ]]; then
-    echo "usage: worktree {add,cd,list,remove,sync} <branch> [<branch>...]
+    echo "usage: worktree add <branch> [<start-point>]
+       worktree {cd,list,remove,sync} <branch> [<branch>...]
        worktree find [-i] <glob>"
     return 1
   elif [[ "$1" == "-h" ]] || [[ "$1" == "--help" ]]; then
-    echo "usage: worktree {add,cd,list,remove,sync} <branch> [<branch>...]
+    echo "usage: worktree add <branch> [<start-point>]
+       worktree {cd,list,remove,sync} <branch> [<branch>...]
        worktree find [-i] <glob>
 
-Manage git worktrees by branch name."
+Manage git worktrees by branch name. A new branch starts from <start-point>,
+e.g. other/main, or origin/main by default."
     return 0
   fi
 
@@ -114,9 +117,33 @@ Manage git worktrees by branch name."
         print -P "%F{red}error:%f missing <branch> argument"
         return 1
       fi
+      if (( $# > 3 )); then
+        print -P "%F{red}error:%f unexpected argument: $4"
+        return 1
+      fi
+      local start=$3
       local root=$(git rev-parse --show-toplevel)
       local wt_dest=${root:h}/${root:t}@${branch}
-      if ! git show-ref --verify --quiet refs/heads/"$branch" && \
+      if [[ -n "$start" ]] && \
+         ! git rev-parse --verify --quiet "$start^{commit}" >/dev/null; then
+        # A <remote>/<branch> start point that hasn't been fetched yet is
+        # fetched now. Remote names can contain slashes, so the longest
+        # matching remote wins.
+        local remote= r
+        for r in $(git remote); do
+          [[ "$start" == "$r"/?* ]] && (( ${#r} > ${#remote} )) && remote=$r
+        done
+        if [[ -n "$remote" ]]; then
+          local rbranch=${start#$remote/}
+          git fetch "$remote" \
+            "+refs/heads/${rbranch}:refs/remotes/${remote}/${rbranch}" || return 1
+        fi
+      fi
+      if [[ -n "$start" ]]; then
+        # An explicit <start-point> always makes a new branch, so this fails
+        # rather than silently ignoring it when the branch already exists.
+        git branch "$branch" "$start" || return 1
+      elif ! git show-ref --verify --quiet refs/heads/"$branch" && \
            ! git show-ref --verify --quiet refs/remotes/origin/"$branch"; then
         git branch "$branch" origin/main || return 1
       fi