Fix modman for "auto-append-gitignore"

magento-hackathon/magento-composer-installer has an option
"auto-append-gitignore" which adds the contents of modman to .gitignore
for you. However, it does this blindly and using a wildcard on a common
directory causes the entire directory to be added, in this case the
shell directory.
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Merge pull request #44 from rbrown/fix-modman

Fix modman for "auto-append-gitignore"