[Xcb] proto: improve diff output

[Daniel Martin]

Hi,

just figured out howto make the git diff output more readable, which I
would like to share with you. Currently, the line reference of a diff
looks like (note the useless string from the licence and the line number
on its own telling nothing where the change really happened):

    @@ -2768,9 +2768,8 @@ authorization from the authors.

After applying the additions, it becomes:

    @@ -2768,9 +2768,8 @@ <event name="ActionMessage" number="9" no-sequence-number="true">


To get this result, you've to:

- add to .gitattributes:
    *.xml diff=xcb

- add to .git/config (or the global config):
    [diff "xcb"]
        xfuncname = "(<\\b(xcb|enum|event|request|struct|union)\\b.*>)"


Cheers,
    Daniel Martin