Teach resolve-ChangeLogs to act as a merge-driver for Git

Teach resolve-ChangeLogs to act as a merge-driver for Git

Reviewed by Eric Seidel.

https://bugs.webkit.org/show_bug.cgi?id=28721

To enable automatic merging of ChangeLog files, use the following command:

git config merge.changelog.driver "resolve-ChangeLogs --merge-driver %O %A %B"

The driver always works in "downstream" merge mode, meaning
it will always put the user's changelog entries on top.

* Scripts/resolve-ChangeLogs:
* .gitattributes: Add instruction to use merge-driver for ChangeLogs

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@54960 268f45cc-cd09-0410-ab3c-d52691b4dbfc