tests: make sed usage in test-unionrepo.t cross-platform

Usage of the 'i' command proves tricky. I tried to write a check-code rule, but
failed.

tests/test-unionrepo.t

@@ -1,149 +1,150 b''
 Test unionrepo functionality
 
 Create one repository
 
   $ hg init repo1
   $ cd repo1
   $ touch repo1-0
   $ echo repo1-0 > f
   $ hg ci -Aqmrepo1-0
   $ touch repo1-1
   $ echo repo1-1 >> f
   $ hg ci -Aqmrepo1-1
   $ touch repo1-2
   $ echo repo1-2 >> f
   $ hg ci -Aqmrepo1-2
   $ hg log --template '{rev}:{node|short}  {desc|firstline}\n'
   2:68c0685446a3  repo1-2
   1:8a58db72e69d  repo1-1
   0:f093fec0529b  repo1-0
   $ tip1=`hg id -q`
   $ cd ..
 
 - and a clone with a not-completely-trivial history
 
   $ hg clone -q repo1 --rev 0 repo2
   $ cd repo2
   $ touch repo2-1
   $ sed '1i\
-  > repo2-1 at top' f > f.tmp
+  > repo2-1 at top
+  > ' f > f.tmp
   $ mv f.tmp f
   $ hg ci -Aqmrepo2-1
   $ touch repo2-2
   $ hg pull -q ../repo1 -r 1
   $ hg merge -q
   $ hg ci -Aqmrepo2-2-merge
   $ touch repo2-3
   $ echo repo2-3 >> f
   $ hg ci -mrepo2-3
   $ hg log --template '{rev}:{node|short}  {desc|firstline}\n'
   4:2f0d178c469c  repo2-3
   3:9e6fb3e0b9da  repo2-2-merge
   2:8a58db72e69d  repo1-1
   1:c337dba826e7  repo2-1
   0:f093fec0529b  repo1-0
   $ cd ..
 
 revisions from repo2 appear as appended / pulled to repo1
 
   $ hg -R union:repo1+repo2 log --template '{rev}:{node|short}  {desc|firstline}\n'
   5:2f0d178c469c  repo2-3
   4:9e6fb3e0b9da  repo2-2-merge
   3:c337dba826e7  repo2-1
   2:68c0685446a3  repo1-2
   1:8a58db72e69d  repo1-1
   0:f093fec0529b  repo1-0
 
 manifest can be retrieved for revisions in both repos
 
   $ hg -R union:repo1+repo2 mani -r $tip1
   f
   repo1-0
   repo1-1
   repo1-2
   $ hg -R union:repo1+repo2 mani -r 4
   f
   repo1-0
   repo1-1
   repo2-1
   repo2-2
 
 files can be retrieved form both repos
 
   $ hg -R repo1 cat repo1/f -r2
   repo1-0
   repo1-1
   repo1-2
 
   $ hg -R union:repo1+repo2 cat -r$tip1 repo1/f
   repo1-0
   repo1-1
   repo1-2
 
   $ hg -R union:repo1+repo2 cat -r4 $TESTTMP/repo1/f
   repo2-1 at top
   repo1-0
   repo1-1
 
 files can be compared across repos
 
   $ hg -R union:repo1+repo2 diff -r$tip1 -rtip
   diff -r 68c0685446a3 -r 2f0d178c469c f
   --- a/f	Thu Jan 01 00:00:00 1970 +0000
   +++ b/f	Thu Jan 01 00:00:00 1970 +0000
   @@ -1,3 +1,4 @@
   +repo2-1 at top
    repo1-0
    repo1-1
   -repo1-2
   +repo2-3
 
 heads from both repos are found correctly
 
   $ hg -R union:repo1+repo2 heads --template '{rev}:{node|short}  {desc|firstline}\n'
   5:2f0d178c469c  repo2-3
   2:68c0685446a3  repo1-2
 
 revsets works across repos
 
   $ hg -R union:repo1+repo2 id -r "ancestor($tip1, 5)"
   8a58db72e69d
 
 annotate works - an indication that linkrevs works
 
   $ hg --cwd repo1 -R union:../repo2 annotate $TESTTMP/repo1/f -r tip
   3: repo2-1 at top
   0: repo1-0
   1: repo1-1
   5: repo2-3
 
 union repos can be cloned ... and clones works correctly
 
   $ hg clone -U union:repo1+repo2 repo3
   requesting all changes
   adding changesets
   adding manifests
   adding file changes
   added 6 changesets with 11 changes to 6 files (+1 heads)
 
   $ hg -R repo3 paths
   default = union:repo1+repo2
 
   $ hg -R repo3 verify
   checking changesets
   checking manifests
   crosschecking files in changesets and manifests
   checking files
   6 files, 6 changesets, 11 total revisions
 
   $ hg -R repo3 heads --template '{rev}:{node|short}  {desc|firstline}\n'
   5:2f0d178c469c  repo2-3
   2:68c0685446a3  repo1-2
 
   $ hg -R repo3 log --template '{rev}:{node|short}  {desc|firstline}\n'
   5:2f0d178c469c  repo2-3
   4:9e6fb3e0b9da  repo2-2-merge
   3:c337dba826e7  repo2-1
   2:68c0685446a3  repo1-2
   1:8a58db72e69d  repo1-1
   0:f093fec0529b  repo1-0