merge-file doc: set conflict-marker-size attribute

When committing a conflict resolution for a merge containing
1f010d6 (doc: use .adoc extension for AsciiDoc files, 2025-01-20)
my pre-commit hook failed because "git diff --check" thought there was
a left over conflict marker in "merge-file.adoc". Fix this by setting
the "conflict-marker-size" attribute as we do for all the other
documentation files that contain example conflict markers.

Signed-off-by: Phillip Wood <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: phillipwood

.gitattributes

@@ -13,6 +13,7 @@ CODE_OF_CONDUCT.md -whitespace
 /mergetools/* text eol=lf
 /t/oid-info/* text eol=lf
 /Documentation/git-merge.adoc conflict-marker-size=32
+/Documentation/git-merge-file.adoc conflict-marker-size=32
 /Documentation/gitk.adoc conflict-marker-size=32
 /Documentation/user-manual.adoc conflict-marker-size=32
 /t/t????-*.sh conflict-marker-size=32