tests/test-histedit-merge-tools.t

tests: use proctutil.stdout.write() instead of print() in test-extension.t

I was debugging this test failure on python3 + chg. I get the following hunk as
test failure:

```
@@ -206,6 +206,18 @@ Check normal command's load order of ext
   4) bar uipopulate
   5) foo reposetup
   5) bar reposetup
+  4) foo uipopulate (chg !)
+  4) bar uipopulate (chg !)
+  4) foo uipopulate (chg !)
+  4) bar uipopulate (chg !)
+  4) foo uipopulate (chg !)
+  4) bar uipopulate (chg !)
+  4) foo uipopulate (chg !)
+  4) bar uipopulate (chg !)
+  4) foo uipopulate (chg !)
+  4) bar uipopulate (chg !)
+  5) foo reposetup (chg !)
+  5) bar reposetup (chg !)
   0:c24b9ac61126
```

After hours of debugging and head scracthing, I figured out that something is
wrong with output flushing. I initially switched the print() statements to
ui.warn() but thanks to Yuya who suggested using procutil.stdout.write()
instead.

Test histedit extension: Merge tools
====================================

Initialization
---------------

  $ . "$TESTDIR/histedit-helpers.sh"

  $ cat >> $HGRCPATH <<EOF
  > [alias]
  > logt = log --template '{rev}:{node|short} {desc|firstline}\n'
  > [extensions]
  > histedit=
  > mockmakedate = $TESTDIR/mockmakedate.py
  > [ui]
  > pre-merge-tool-output-template='pre-merge message for {node}\n'
  > EOF

Merge conflict
--------------

  $ hg init r
  $ cd r
  $ echo foo > file
  $ hg add file
  $ hg ci -m "First" -d "1 0"
  $ echo bar > file
  $ hg ci -m "Second" -d "2 0"

  $ hg logt --graph
  @  1:2aa920f62fb9 Second
  |
  o  0:7181f42b8fca First
  

Invert the order of the commits, but fail the merge.
  $ hg histedit --config ui.merge=false --commands - 2>&1 <<EOF | fixbundle
  > pick 2aa920f62fb9 Second
  > pick 7181f42b8fca First
  > EOF
  merging file
  pre-merge message for b90fa2e91a6d11013945a5f684be45b84a8ca6ec
  merging file failed!
  Fix up the change (pick 7181f42b8fca)
  (hg histedit --continue to resume)

  $ hg histedit --abort | fixbundle
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved

Invert the order of the commits, and pretend the merge succeeded.
  $ hg histedit --config ui.merge=true --commands - 2>&1 <<EOF | fixbundle
  > pick 2aa920f62fb9 Second
  > pick 7181f42b8fca First
  > EOF
  merging file
  pre-merge message for b90fa2e91a6d11013945a5f684be45b84a8ca6ec
  7181f42b8fca: skipping changeset (no changes)