Add more document types to .gitattributes

Marking binary document types as such prevents errors that I see
locally, like

  $ git log -S getWordCompatibilityMode
  E: unsupported filetype sw/qa/uitest/data/tdf168070_2.docx
  fatal: unable to read files to diff

This also speeds up the search.

RTF files are marked 'diff', because they could in theory be considered
plaintext markup; maybe marking them also binary would be better.
Having them here helps prevents this:

  E: unsupported filetype C:\Users\mikek\AppData\Local\Temp/git-blob-a26856/CVE-2005-2971-1.rtf
  fatal: unable to read files to diff

Change-Id: I5da14b041f828e2ed1ff6c0acc9722e252802ef9
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/191591
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

.gitattributes

@@ -1,4 +1,13 @@
 *.doc binary
+*.ppt binary
+*.xls binary
 *.sdw binary
 *.jcs binary
+*.docx binary
+*.ppts binary
+*.xlsx binary
+*.odp binary
+*.ods binary
+*.odt binary
+*.rtf diff
 schema/*/* export-ignore