fix(contrib): fix scm-prompt triggering chpwd hooks during directory traversal (#1325)

Summary:
Currently, trying to open a new tab in Ghostty within a repository
causes the tab to open in the repository root instead of the current working
directory. This commit fixes the issue.

When scm-prompt.sh walks up the directory tree to find the root of the
repository (.sl, .git, or .hg), it uses `builtin cd -P`. In Zsh,
executing `cd` (even inside a subshell) triggers all functions registered
in `chpwd_functions`.

This causes unexpected side effects if a `chpwd` hook performs operations
that bypass standard output capture. For example, Ghostty's shell integration
registers a `chpwd` hook that writes an OSC 7 escape sequence directly to a
persistent TTY file descriptor (/dev/tty) to report the current working
directory. Because this writes to the TTY directly, the traversal performed by
`scm-prompt.sh` leaks temporary working directories to the terminal emulator,
causing Ghostty to open new tabs in the repository root rather than the actual
current working directory.

By localizing and clearing `chpwd_functions` within the subshell before executing
`cd`, we prevent these hooks from firing during the directory traversal, resolving
the side effects while keeping the traversal logic intact.

Pull Request resolved: #1325

Test Plan:
Run this script.  Before this commit, it will print the path to the repository
root.  After this commit, it will print the current working directory.

```sh
#!/usr/bin/env zsh

# Clear previous output
rm -f /tmp/ghostty_bug_repro.txt
rm -f /tmp/ghostty_log.txt

# Create a self-contained temporary home/zdotdir for this test
REPRO_ZDOTDIR=$(mktemp -d)

# Write out the minimal reproducible .zshrc
cat << 'ZSHRC_EOF' > "$REPRO_ZDOTDIR/.zshrc"
# We need prompt_subst to allow function execution in prompt
setopt prompt_subst

# Load the buggy sapling prompt (assuming it's installed at this path)
source ~/workspace/github.com/facebook/sapling/eden/scm/contrib/scm-prompt.sh

# Set the prompt to trigger the bug
export PS1='$(_scm_prompt) %~ %# '

# To test Ghostty OSC 7 tracking, we need Ghostty shell integration
source "$GHOSTTY_RESOURCES_DIR/shell-integration/zsh/ghostty-integration"
ZSHRC_EOF

# Launch Ghostty with logging and our minimal zshrc
# We use ZDOTDIR to point zsh to our custom zshrc
GHOSTTY_LOG=stderr ZDOTDIR="$REPRO_ZDOTDIR" /Users/bytedance/workspace/github.com/ghostty-org/ghostty/zig-out/Ghostty.app/Contents/MacOS/ghostty > /tmp/ghostty_log.txt 2>&1 &
GHOSTTY_PID=$!

# Give it a moment to start
sleep 1

# Run the AppleScript to drive the UI
osascript << 'APPLESCRIPT'
tell application "System Events"
    tell application "/Users/bytedance/workspace/github.com/ghostty-org/ghostty/zig-out/Ghostty.app" to activate
    delay 1

    -- cd to a deeply nested directory in the sapling repo
    keystroke "cd ~/workspace/github.com/facebook/sapling/eden/scm/"
    keystroke return
    delay 1

    -- open a new tab
    keystroke "t" using command down
    delay 1

    -- get the pwd in the new tab and write it to a file
    keystroke "pwd > /tmp/ghostty_bug_repro.txt"
    keystroke return
    delay 1

    -- quit Ghostty
    keystroke "q" using command down
end tell
APPLESCRIPT

# Read the output
echo "The new tab's working directory was:"
cat /tmp/ghostty_bug_repro.txt

echo "Ghostty PID was $GHOSTTY_PID"

echo "Ghostty logs are in: /tmp/ghostty_log.txt"

# Clean up
rm -rf "$REPRO_ZDOTDIR"
```

Reviewed By: quark-zju

Differential Revision: D106557429

fbshipit-source-id: 4b2965dcf49c03deb543af993c1273f3f18057a3

Co-authored-by: Trae <trae@bytedance.com>

Author: 2 people

eden/scm/contrib/scm-prompt.sh

@@ -257,7 +257,7 @@ _scm_prompt() {
     fi
     [[ "$dir" = "/" ]] && break
     # portable "realpath" equivalent
-    dir="$(builtin cd -P "$dir/.." >/dev/null && builtin echo "$PWD")"
+    dir="$(chpwd_functions=(); chpwd() { :; }; builtin cd -P "$dir/.." >/dev/null && builtin echo "$PWD")"
   done
 
   if [[ -n "$br" ]]; then