fix backtrace printed when cannot get lock.
change lock error handling code so exceptions have useful info and
exception handling in one place.
add test case for when cannot get lock.
#!/bin/sh
hg init
cat > .hg/hgrc <<EOF
[encode]
*.gz = gunzip
[decode]
*.gz = gzip
EOF
echo "this is a test" | gzip > a.gz
hg add a.gz
hg ci -m "test" -d "1000000 0"
echo %% no changes
hg status
touch a.gz
echo %% no changes
hg status
echo %% uncompressed contents in repo
hg debugdata .hg/data/a.gz.d 0
echo %% uncompress our working dir copy
gunzip < a.gz
rm a.gz
hg co
echo %% uncompress our new working dir copy
gunzip < a.gz