vimrc: fix space-before-tab handling in diffs.

Don't flag a single space at the beginning of the line before a tab if
viewing a diff, as we might be looking at tab-indented code.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
vimrc: fix perl whitespace rules to handle pod.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
vimrc: use standard pattern to clear trailing whitespace.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>