Add below in settings.json
"git-graph.customEmojiShortcodeMappings": [
{"shortcode": ":butterfly:", "emoji":"🦋"},
{"shortcode": ":up:", "emoji":"🆙"},
{"shortcode": ":beer:", "emoji":"🍺"},
{"shortcode": ":test_tube:", "emoji":"🧪"},
{"shortcode": ":warning:", "emoji": "⚠️"}
]