attr: enable incomplete-line whitespace error for this project

Now "git diff --check" and "git apply --whitespace=warn/fix" learned
incomplete line is a whitespace error, enable them for this project
to prevent patches to add new incomplete lines to our source to both
code and documentation files.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitattributes

@@ -1,13 +1,13 @@
 * whitespace=!indent,trail,space
-*.[ch] whitespace=indent,trail,space diff=cpp
-*.sh whitespace=indent,trail,space text eol=lf
+*.[ch] whitespace=indent,trail,space,incomplete diff=cpp
+*.sh whitespace=indent,trail,space,incomplete text eol=lf
 *.perl text eol=lf diff=perl
 *.pl text eof=lf diff=perl
 *.pm text eol=lf diff=perl
 *.py text eol=lf diff=python
 *.bat text eol=crlf
 CODE_OF_CONDUCT.md -whitespace
-/Documentation/**/*.adoc text eol=lf
+/Documentation/**/*.adoc text eol=lf whitespace=trail,space,incomplete
 /command-list.txt text eol=lf
 /GIT-VERSION-GEN text eol=lf
 /mergetools/* text eol=lf