test-convert-svn-branches: filter unwanted output

Windows cmd.exe emits a different message than sh:
"""
foobar is not recognized as an internal or external command,
operable program or batch file.
"""
Filter it.

tests/test-convert-svn-branches

@@ -25,5 +25,5 @@ hg tags -q
 cd ..
 
 echo '% test hg failing to call itself'
-HG=foobar hg convert svn-repo B-hg 2>&1 | grep -v foobar
+HG=foobar hg convert svn-repo B-hg 2>&1 | grep itself
 

tests/test-convert-svn-branches.out

@@ -49,5 +49,4 @@ old                            9:
 old2                           8:
 tip
 % test hg failing to call itself
-initializing destination B-hg repository
 abort: Mercurial failed to run itself, check hg executable is in PATH