test-bisect: test handling of invalid commands without relying on sh

tests/test-bisect

@@ -86,8 +86,9 b' hg bisect -g'
 set +e
 
 echo % test invalid command
+# assuming that the shell returns 127 if command not found ...
 hg bisect -r
-hg bisect --command './foobar' 2>&1 | sed 's|\(/bin/sh: \./foobar:\).*|\1 not found|'
+hg bisect --command 'exit 127'
 
 echo % test bisecting command
 cat > script.py <<EOF

tests/test-bisect.out

@@ -304,8 +304,7 b' date: Thu Jan 01 00:00:06 1970 +0'
 summary:     msg 6
 
 % test invalid command
-/bin/sh: ./foobar: not found
-abort: failed to execute ./foobar
+abort: failed to execute exit 127
 % test bisecting command
 Testing changeset 15:e7fa0811edb0 (31 changesets remaining, ~4 tests)
 1 files updated, 0 files merged, 0 files removed, 0 files unresolved