Bug #252727 - Use `git diff-index --name-only --diff-filter=M HEAD` instead

Bug #252727 - Use `git diff-index --name-only --diff-filter=M HEAD` instead
of `git ls-files -m --with-tree=HEAD` since the latter doesn't behave
like we want for files that have been added to the index. Also, use `git
diff-index` instead of `git diff` since the latter is considered a high-level
"porcelain" command which means that it's interface may not be reliable.
(trunk r12359)

svn path=/main/branches/2.1.6/; revision=12430