check-code: replace heredocs in unified tests

Heredocs are usually fed to other commands and
shouldn't follow the standard conventions of shell
commands.

This restores the old behaviour of how heredocs
were handled in old-style test files.

contrib/check-code.py

@@ -116,6 +116,7 b' for i in [0, 1]:'
         utestpats[i].append((p, m))
 
 utestfilters = [
+    (r"<<(\S+)((.|\n)*?\n  > \1)", rephere),
     (r"( *)(#([^\n]*\S)?)", repcomment),
 ]
 

tests/test-check-code-hg.t

@@ -162,19 +162,6 b''
   tests/autodiff.py:0:
    >         ui.write('data lost for: %s\n' % fn)
    warning: unwrapped ui message
-  tests/test-convert-mtn.t:0:
-   >   > function get_passphrase(keypair_id)
-   don't use 'function', use old style
-  tests/test-import-git.t:0:
-   >   > Mc\${NkU|\`?^000jF3jhEB
-   ^ must be quoted
-  tests/test-import.t:0:
-   >   > diff -Naur proj-orig/foo proj-new/foo
-   don't use 'diff -N'
-   don't use 'diff -N'
-  tests/test-schemes.t:0:
-   >   > z = file:\$PWD/
-   don't use $PWD, use `pwd`
   tests/test-ui-color.py:0:
    > testui.warn('warning\n')
    warning: unwrapped ui message