feat: resolve a relative FORGIT_WORKTREE_ADD_DIR per repository

A relative FORGIT_WORKTREE_ADD_DIR is resolved against the root of the
main worktree on every invocation, so a single setting such as
`.worktrees` is valid in every repository. Absolute paths keep their
meaning.

The variable previously only took a full path, so users resolved the
repository root in their shell startup files, which runs git outside a
repository on every new shell and freezes the path for the whole
session.

README.md

@@ -381,7 +381,7 @@ export FORGIT_LOG_FZF_OPTS='
 | `FORGIT_FULLSCREEN_CONTEXT`                 | lines of diff context in full-screen mode                                                                                        | 10                                            |
 | `FORGIT_DIR_VIEW`                           | command used to preview directories                                                                                              | `tree` if available, otherwise `find`         |
 | `FORGIT_CLEAN_LIST_FILES_OPTS`              | arguments passed to `git ls-files` together with `--others` to determine which files are shown when invoking `forgit clean`      |                                               |
-| `FORGIT_WORKTREE_ADD_DIR`                   | directory where new worktrees are created                                                                                        | `<repo-root>/.wt`                             |
+| `FORGIT_WORKTREE_ADD_DIR`                   | directory where new worktrees are created (a relative path is resolved against the main worktree root)                           | `.wt`                                         |
 | `FORGIT_CHECKOUT_BRANCH_AUTO_CREATE_BRANCH` | prevents forgit from auto creating a new branch with `gcb <BRANCH_NAME>` when `<BRANCH_NAME>` does not exist when set to `false` |                                               |
 | `FORGIT_SWITCH_AUTO_CREATE_BRANCH`          | prevents forgit from auto creating a new branch with `gsw <BRANCH_NAME>` when `<BRANCH_NAME>` does not exist when set to `false` |                                               |
 

bin/git-forgit

@@ -1583,6 +1583,20 @@ _forgit_main_worktree_root() {
     git worktree list --porcelain | sed -n 's/^worktree //p;q'
 }
 
+# Get the directory in which new worktrees are created
+# A relative FORGIT_WORKTREE_ADD_DIR is resolved against the root of the main
+# worktree, so that a single setting is valid in every repository. Anchoring it
+# to the main worktree rather than the current one also keeps adding a worktree
+# from a linked worktree from nesting the directory any deeper.
+_forgit_worktree_add_dir() {
+    local dir="${FORGIT_WORKTREE_ADD_DIR:-.wt}"
+    if [[ $dir == /* ]]; then
+        echo "$dir"
+    else
+        echo "$(_forgit_main_worktree_root)/$dir"
+    fi
+}
+
 # Parse git worktree list --porcelain output and format it for display
 # Output format: [XY] /path/to/worktree (branch) 3 hours ago
 #   X: '*' = current worktree, ' ' = other
@@ -1740,8 +1754,8 @@ _forgit_worktree_delete() {
 _forgit_worktree_add() {
     _forgit_inside_git_repo || return 1
 
-    # Default to main worktree root, not current worktree's root
-    local wt_dir="${FORGIT_WORKTREE_ADD_DIR:-$(_forgit_main_worktree_root)/.wt}"
+    local wt_dir
+    wt_dir=$(_forgit_worktree_add_dir)
 
     # Two or more arguments: pass through to git directly
     if [[ $# -ge 2 ]]; then

tests/worktree.test.sh

@@ -0,0 +1,49 @@
+#!/usr/bin/env bash
+
+function set_up_before_script() {
+    source bin/git-forgit
+
+    # Ignore global git config files
+    export GIT_CONFIG_SYSTEM=/dev/null
+    export GIT_CONFIG_GLOBAL=/dev/null
+
+    unset FORGIT_WORKTREE_ADD_DIR
+
+    # Create a temporary git repository for testing
+    MAIN_WORKTREE_ROOT="$(bashunit::temp_dir)/main"
+    git init -q "$MAIN_WORKTREE_ROOT"
+    cd "$MAIN_WORKTREE_ROOT" || return 1
+    git config user.email "test@example.com"
+    git config user.name "Test User"
+    echo "initial" >README.md
+    git add README.md
+    git commit -q -m "Initial commit"
+}
+
+function test_worktree_add_dir_defaults_to_dot_wt_in_main_worktree_root() {
+    assert_same "$MAIN_WORKTREE_ROOT/.wt" "$(_forgit_worktree_add_dir)"
+}
+
+function test_worktree_add_dir_resolves_a_relative_path_against_main_worktree_root() {
+    assert_same "$MAIN_WORKTREE_ROOT/.worktrees" \
+        "$(FORGIT_WORKTREE_ADD_DIR=.worktrees _forgit_worktree_add_dir)"
+}
+
+function test_worktree_add_dir_resolves_a_nested_relative_path() {
+    assert_same "$MAIN_WORKTREE_ROOT/build/wt" \
+        "$(FORGIT_WORKTREE_ADD_DIR=build/wt _forgit_worktree_add_dir)"
+}
+
+function test_worktree_add_dir_uses_an_absolute_path_as_is() {
+    assert_same "/somewhere/else" \
+        "$(FORGIT_WORKTREE_ADD_DIR=/somewhere/else _forgit_worktree_add_dir)"
+}
+
+function test_worktree_add_dir_resolves_a_relative_path_from_a_linked_worktree() {
+    local linked_worktree="$MAIN_WORKTREE_ROOT/.worktrees/linked"
+    git worktree add -q -b linked "$linked_worktree" >/dev/null 2>&1
+    cd "$linked_worktree" || return 1
+
+    assert_same "$MAIN_WORKTREE_ROOT/.worktrees" \
+        "$(FORGIT_WORKTREE_ADD_DIR=.worktrees _forgit_worktree_add_dir)"
+}