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
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
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% expect authorization error: must have authorized user
abort: authorization failed
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remote: adding changesets
remote: adding manifests
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remote: added 1 changesets with 1 changes to 1 files
remote: changegroup hook: HG_NODE=ba677d0156c1196c1a699fa53f390dcfc3ce3872 HG_SOURCE=serve HG_URL=remote:http
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abort: authorization failed
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% expect authorization error: some users denied, users must be authenticated
abort: authorization failed
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% serve errors