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.
adding a
adding b
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
created new head
% Rename is tracked
2:rename A
diff --git a/a b/a-renamed
rename from a
rename to a-renamed
% Rebase the revision containing the rename
% Rename is not lost
2:rename A
diff --git a/a b/a-renamed
rename from a
rename to a-renamed
adding a
adding b
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
created new head
% Copy is tracked
2:copy A
diff --git a/a b/a-copied
copy from a
copy to a-copied
% Rebase the revision containing the copy
% Copy is not lost
2:copy A
diff --git a/a b/a-copied
copy from a
copy to a-copied