.gitattributes: add conflict-marker-size for relevant files

Some files in git.git contain lines that look like conflict markers,
either in examples or tests, or in the case of Documentation/gitk.txt
because of the asciidoc heading.

Having conflict markers the same length as the actual content can be
confusing for humans, and is impossible to handle for tools like 'git
rerere'.  Work around that by setting the 'conflict-marker-size'
attribute for those files to 32, which makes the conflict markers
unambiguous.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitattributes

@@ -9,3 +9,7 @@
 /command-list.txt eol=lf
 /GIT-VERSION-GEN eol=lf
 /mergetools/* 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
+/t/t????-*.sh conflict-marker-size=32