xx-hg-merge: command not found

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/xx-hg-merge - from package: xxdiff-scripts

You may install the required package with command apt-get install {package}

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