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.
$ hg init a
$ hg clone a b
updating to branch default
0 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd a
$ echo '[paths]' >> .hg/hgrc
$ echo 'dupe = ../b' >> .hg/hgrc
$ hg in dupe
comparing with .*/test-paths\.t/b
no changes found
[1]
$ cd ..
$ hg -R a in dupe
comparing with .*/test-paths\.t/b
no changes found
[1]