demos: add automated demo of cloning Git repo

The automation is mostly copied from
https://github.com/arxanas/git-branchless/tree/master/demos, so big
thanks to @arxanas for writing that!

README.md

@@ -23,11 +23,18 @@ well-known).
 Features:
 
  * **Compatible with Git**
-
+   
    Jujutsu has two backends. One of them is a Git backend (the other is a
    native one). This lets you use Jujutsu as an alternative interface to Git.
    The commits you create will look like regular Git commits. You can always
    switch back to Git.
+   
+   <details>
+   <summary>Demo</summary>
+   <a href="https://asciinema.org/a/3LEDzGEmEGnf0sTsvyHm41Jgu" target="_blank">
+   <img src="https://asciinema.org/a/3LEDzGEmEGnf0sTsvyHm41Jgu.svg" />
+   </a>
+   </details>
 
  * **The working copy is automatically committed**
 

demos/demo_git_compat.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+set -euo pipefail
+. "$(dirname "$0")"/demo_helpers.sh
+parse_args "$@"
+
+new_tmp_dir
+
+run_demo '
+expect_prompt
+run_command "# Clone a Git repo:"
+expect_prompt
+run_command "jj git clone https://github.com/octocat/Hello-World"
+expect_prompt
+run_command "cd Hello-World"
+expect_prompt
+run_command "# Inspect it:"
+expect_prompt
+run_command "jj log"
+expect_prompt
+run_command "jj diff -r b1"
+expect_prompt
+run_command "# The repo is backed by the actual Git repo:"
+expect_prompt
+run_command "git --git-dir=.jj/store/git log --graph --all --decorate --oneline"
+'

demos/demo_helpers.sh

@@ -0,0 +1,77 @@
+#!/bin/bash
+set -euo pipefail
+BASE_DIR=$(realpath "$(dirname "$0")")
+
+UPLOAD=false
+PREVIEW=false
+DEBUG=false
+parse_args() {
+    for arg in "$@"; do
+        case "$arg" in
+        -h|--help)
+            echo 'Run a given demo.
+Arguments:
+  --preview: Preview the asciicast.
+  --upload: Upload to asciinema (after previewing, if necessary).
+  --debug: Show the asciicast as it is being recorded. Note that what you see
+           will not be exactly the same as what is recorded.
+'
+            exit
+            ;;
+        --upload)
+            UPLOAD=true
+            ;;
+        --preview)
+            PREVIEW=true
+            ;;
+        --debug)
+            DEBUG=true
+            ;;
+        *)
+            echo "Unrecognized argument: $arg"
+            exit 1
+            ;;
+        esac
+    done
+}
+
+new_tmp_dir() {
+    local dirname
+    dirname=$(mktemp -d)
+    mkdir -p "$dirname"
+    cd "$dirname"
+    trap "rm -rf '$dirname'" EXIT
+}
+
+run_demo() {
+    local expect_script="$1"
+    expect_script=$(printf "source $BASE_DIR/demo_helpers.tcl
+spawn asciinema rec -c \"PS1='$ ' bash --norc\"
+expect_prompt
+%s
+quit_and_dump_asciicast_path
+" "$expect_script")
+     
+    if [[ "$DEBUG" == true ]]; then
+        echo "$expect_script" | /usr/bin/env expect
+        return
+    fi
+
+    echo "Recording demo (terminal size is $(tput cols)x$(tput lines))..."
+    if [[ "$PREVIEW" == 'false' ]]; then
+        echo '(Pass --preview to play the demo automatically once done)'
+    fi
+    local asciicast_path
+    asciicast_path=$(echo "$expect_script" | /usr/bin/env expect | tail -1)
+    echo "$asciicast_path"
+
+    if [[ "$PREVIEW" == 'true' ]]; then
+        asciinema play "$asciicast_path"
+    fi
+    if [[ "$UPLOAD" == 'true' ]]; then
+        if [[ "$PREVIEW" == 'true' ]] && ! confirm "Upload?"; then
+            return
+        fi
+        : asciinema upload "$asciicast_path"
+    fi
+}

demos/demo_helpers.tcl

@@ -0,0 +1,26 @@
+set send_human {0.1 0.3 1 0.05 1}
+set timeout 2
+
+proc expect_prompt {} {
+    expect "$ "
+}
+
+proc run_command {cmd} {
+    send -h "$cmd"
+    send "\r"
+    expect -timeout 1
+}
+
+proc quit_and_dump_asciicast_path {} {
+    set CTRLC \003
+    set CTRLD \004
+    set ESC \033
+
+    send $CTRLD
+    expect "asciinema: recording finished"
+    sleep 1
+    send $CTRLC
+    expect -re "asciicast saved to (.+)$ESC.*\r" {
+        send_user "$expect_out(1,string)\n"
+    }
+}