tests: don't match blank output lines as regexes in unified tests
Currently, the following unified test will pass:
$ echo foo
A blank output line (a line containing just two spaces) will match any
output.
The patch modifies the unified test runner to ignore empty strings
strings when do regular expression matching.
#!/bin/sh
"$TESTDIR/hghave" fifo || exit 80
hg init
mkfifo p
hg serve --stdio < p &
P=$!
(echo lock; echo addchangegroup; sleep 5) > p &
Q=$!
sleep 3
kill -HUP $P
wait
echo .hg/* .hg/store/*