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.
$ echo "[extensions]" >> $HGRCPATH
$ echo "bookmarks=" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo qqq>qqq.txt
add file
$ hg add
adding qqq.txt
commit first revision
$ hg ci -m 1
set bookmark
$ hg book test
$ echo www>>qqq.txt
commit second revision
$ hg ci -m 2
set bookmark
$ hg book test2
update to -2
$ hg update -r -2
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo eee>>qqq.txt
commit new head
$ hg ci -m 3
created new head
bookmarks updated?
$ hg book
test 1:25e1ee7a0081
test2 1:25e1ee7a0081
strip to revision 1
$ hg strip 1
saved backup bundle to .*
list bookmarks
$ hg book
* test 1:8cf31af87a2b
* test2 1:8cf31af87a2b