git-undo: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/git-undo - from package: git-extras

You may install the required package with command apt-get install {package}
Error in other languages:
git-undo: bevel nie gevind nie
git-undo: comandă negăsită
git-undo: comando não encontrado
git-undo: commande introuvable
git-undo: command not found
git-undo: käsku ei ole
git-undo: không tìm thấy lệnh
git-undo: komanda nerasta
git-undo: Kommando nicht gefunden.
git-undo: kommandot finns inte
git-undo: komut yok
git-undo: nie znaleziono polecenia
git-undo: níor aimsíodh an t-ordú
git-undo: no se encontró la orden
git-undo: no s'ha trobat l'ordre
git-undo: opdracht niet gevonden
git-undo: parancs nem található
git-undo: perintah tidak ditemukan
git-undo: príkaz nenájdený
git-undo: příkaz nenalezen
git-undo: команда не найдена
git-undo: командата не е открита
git-undo: 命令找不到
git-undo: コマンドが見つかりません