run-tests.py: fix diff output when test-foo.out doesn't exist.
Previously, if test-foo.out doesn't exist and test-foo outputs
something, the first line of the diff will look like this:
-+changeset: 0:0acdaf898367
(note the leading "-")
changeset: 0:0acdaf898367
tag: tip
user: test
date: Mon Jan 12 13:46:40 1970 +0000
summary: test
changeset: 1:c5c60883086f
tag: tip
user: test
date: Mon Jan 12 13:46:40 1970 +0000
summary: Added tag bleah for changeset 0acdaf8983679e0aac16e811534eb49d7ee1f2b4
changeset: 0:0acdaf898367
tag: bleah
user: test
date: Mon Jan 12 13:46:40 1970 +0000
summary: test
abort: working copy of .hgtags is changed (please commit .hgtags manually)
failed
use of 'hg tag NAME [REV]' is deprecated, please use 'hg tag [-r REV] NAME' instead
abort: use only one form to specify the revision
failed
use of 'hg tag NAME [REV]' is deprecated, please use 'hg tag [-r REV] NAME' instead
0acdaf8983679e0aac16e811534eb49d7ee1f2b4 bleah
0acdaf8983679e0aac16e811534eb49d7ee1f2b4 bleah0
c5c60883086f5526bd3e36814b94a73a4e75e172 bleah1
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
0acdaf8983679e0aac16e811534eb49d7ee1f2b4 foobar
c5c60883086f5526bd3e36814b94a73a4e75e172 bleah1
abort: '\n' cannot be used in a tag name
abort: ':' cannot be used in a tag name