bash_completion: try to use bash3 features if they're available
This does NOT break completion for bash 2.05b (I haven't tested
earlier versions)
#!/bin/sh
mkdir t
cd t
hg init
echo import > port
hg add port
hg commit -m 0 -u spam -d '0 0'
echo export >> port
hg commit -m 1 -u eggs -d '1 0'
echo export > port
echo vaportight >> port
echo 'import/export' >> port
hg commit -m 2 -u spam -d '2 0'
echo 'import/export' >> port
hg commit -m 3 -u eggs -d '3 0'
head -3 port > port1
mv port1 port
hg commit -m 4 -u spam -d '4 0'
hg grep port port
hg grep -enu port port
hg grep import port