fix: make git merge2 work across worktrees

Checkout of the target branch fails when it is already used by another
worktree. Detect that worktree and merge there instead.

git/.gitconfig

@@ -73,7 +73,8 @@
   pt = push origin --tags
   ph = push origin HEAD
   pht = push origin --tags
-  merge2 = !sh -c '_CURRENT_BRANCH=$(git symbolic-ref --short HEAD) && git checkout $1 && git merge $_CURRENT_BRANCH && git checkout $_CURRENT_BRANCH' -
+  # 把当前分支合进目标分支后回到当前分支；目标已被其它 worktree 占用时在那边 merge
+  merge2 = !$HOME/.dotfile/git/git-merge2.sh
   bg = "!f() { git branch | grep $1; }; f"
   delbr = "!f(){ git branch -D $(git branch | grep \"\b$1\"); };f"
   can = commit --amend --no-edit

git/git-merge2.sh

@@ -0,0 +1,72 @@
+#!/bin/bash
+# 把当前分支 merge 进目标分支；目标已在其它 worktree 时直接在那边 merge
+set -euo pipefail
+
+usage() {
+    cat <<'EOF'
+用法: git merge2 <branch> [git merge 参数...]
+
+把当前分支合进 <branch>，然后留在当前分支。
+
+若 <branch> 已在其它 worktree 里 checkout，则在那个 worktree 里 merge，
+不会在当前目录 checkout（避免 "already used by worktree"）。
+EOF
+}
+
+if [[ "${1:-}" == "-h" || "${1:-}" == "--help" ]]; then
+    usage
+    exit 0
+fi
+
+if [[ $# -lt 1 ]]; then
+    usage >&2
+    exit 1
+fi
+
+current="$(git symbolic-ref --short HEAD)"
+target="$1"
+shift
+
+if [[ "$current" == "$target" ]]; then
+    echo "已经在 $target 上" >&2
+    exit 1
+fi
+
+# porcelain 含主 worktree；%(worktreepath) 只填 linked worktree，不能用
+worktree_for_branch() {
+    local branch="$1"
+    local wt=""
+    local line
+    while IFS= read -r line || [[ -n "$line" ]]; do
+        case "$line" in
+            worktree\ *)
+                wt="${line#worktree }"
+                ;;
+            "branch refs/heads/${branch}")
+                printf '%s\n' "$wt"
+                return 0
+                ;;
+            "")
+                wt=""
+                ;;
+        esac
+    done < <(git worktree list --porcelain)
+    return 1
+}
+
+# alias / 子进程会继承当前 worktree 的 GIT_DIR，必须清掉才能 git -C 到另一个 worktree
+merge_in_worktree() {
+    local wt="$1"
+    shift
+    env -u GIT_DIR -u GIT_WORK_TREE git -C "$wt" merge "$@"
+}
+
+wt="$(worktree_for_branch "$target" || true)"
+if [[ -n "$wt" ]]; then
+    echo "在 worktree 中 merge $current -> $target: $wt"
+    merge_in_worktree "$wt" "$current" "$@"
+else
+    git checkout "$target"
+    git merge "$current" "$@"
+    git checkout "$current"
+fi