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
echo 123 > a
echo 123 > c
hg add a c
hg commit -m "first" -d "1000000 0" a c
echo 123 > b
echo %% should show b unknown
hg status
echo 12 > c
echo %% should show b unknown and c modified
hg status
hg add b
echo %% should show b added and c modified
hg status
hg rm a
echo %% should show a removed, b added and c modified
hg status
hg revert a
echo %% should show b added and c modified
hg status
hg revert b
echo %% should show b unknown and c modified
hg status
hg revert c
echo %% should show b unknown
hg status
echo %% should show a b and c
ls
true