.gitattributes: more *.txt -> *.adoc updates

All Documentation files now end in .adoc.  Update the entries for
git-merge.adoc, gitk.adoc, and user-manual.adoc to properly set the
conflict-marker-size attribute.

Signed-off-by: Todd Zullinger <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: tmzullinger

.gitattributes

@@ -12,7 +12,7 @@ CODE_OF_CONDUCT.md -whitespace
 /GIT-VERSION-GEN text eol=lf
 /mergetools/* text eol=lf
 /t/oid-info/* text eol=lf
-/Documentation/git-merge.txt conflict-marker-size=32
-/Documentation/gitk.txt conflict-marker-size=32
-/Documentation/user-manual.txt conflict-marker-size=32
+/Documentation/git-merge.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