PathSource::list_files doesn't look at .gitignore if there's no package in the git root

[alexcrichton]

Currently PathSource::list_files looks for packages where Repository::open will succeed, but this will never succeed if there is never a package at the root of a git repository. This means that filters such as pruning everything in .gitignore from the file listings will not work because no git repository is detected.

To replicate this behavior:

• Create a git repository and change directories into it
• cargo new a
• cargo new b
• Edit b to depend on a (as a path dependency)
• Edit a to have a build script which creates the file a/foo in the git repository
• Add a/foo to the repo's .gitignore

On re-running cargo build in the directory for b it will spuriously rebuild a too frequently as it doesn't know that it should ignore a/foo.

[alexcrichton]

Also note that there is a comment to this effect, it just turned out to be correct!