.gitattributes: fix end-of-lines

The test suite does actually not pass when files are checked out with
CR/LF line endings, so we have to force them to LF-only.

For good measure, we simply force all source files to LF, also to make
it easier for Git for Windows contributors to contribute to upstream
Git's source code.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

.gitattributes

@@ -1,3 +1,4 @@
 * whitespace=!indent,trail,space
-*.[ch] whitespace=indent,trail,space diff=cpp
-*.sh whitespace=indent,trail,space
+*.[ch] whitespace=indent,trail,space diff=cpp eol=lf
+*.sh whitespace=indent,trail,space eol=lf
+*.{perl,pm,txt} eol=lf