D6369: templatekw: make {file_*} compare to both merge parents (issue4292)

[Yuya Nishihara]

Test failures:

```
--- /home/yuya/work/hghacks/mercurial-review/tests/test-convert-bzr-merges.t
+++ /home/yuya/work/hghacks/mercurial-review/tests/test-convert-bzr-merges.t.err
@@ -59,7 +59,7 @@
   $ glog -R source-hg
   o    5 at source "(octopus merge fixup)" files+: [], files-: [], files: [renamed]
   |\
-  | o    4 at source "Merged branches" files+: [file-branch1 file-branch2 renamed], files-: [rename_me], files: [file]
+  | o    4 at source "Merged branches" files+: [file-branch2 renamed], files-: [rename_me], files: []
   | |\
   o---+  3 at source-branch2 "Added brach2 file" files+: [file-branch2 renamed], files-: [rename_me], files: []
    / /
@@ -154,7 +154,7 @@
   $ glog -R hg2hg
   @    5 at source "(octopus merge fixup)" files+: [], files-: [], files: []
   |\
-  | o    4 at source "Merged branches" files+: [file-branch1 file-branch2 renamed], files-: [rename_me], files: [file]
+  | o    4 at source "Merged branches" files+: [file-branch2 renamed], files-: [rename_me], files: []
   | |\
   o---+  3 at source-branch2 "Added brach2 file" files+: [file-branch2 renamed], files-: [rename_me], files: []
    / /

ERROR: test-convert-bzr-merges.t output changed
!......................................ss..........
--- /home/yuya/work/hghacks/mercurial-review/tests/test-convert-bzr.t
+++ /home/yuya/work/hghacks/mercurial-review/tests/test-convert-bzr.t.err
@@ -147,7 +147,7 @@
   1 Editing b
   0 Merged improve branch
   $ glog -R source-hg
-  o    3 at source "Merged improve branch" files+: [], files-: [], files: [b]
+  o    3 at source "Merged improve branch" files+: [], files-: [], files: []
   |\
   | o  2 at source-improve "Editing b" files+: [], files-: [], files: [b]
   | |

ERROR: test-convert-bzr.t output changed
```