Factor in .gitignore for build cmd freshness

Factor in .gitignore for build cmd freshness

When testing the freshness of a build command, the fingerprint of an entire
package is generated. The current method of fingerprinting a path source is to
just stat() the entire tree and look at mtimes. This works fine when the build
command places *all* output in the build directory.

Some systems, like autotools, place *most* output in the build directory and
some output in the source directory, however (see spidermonkey). In this case
the coarse-grained "consider all files in a directory as input" is too painful
for the build command as the package will forever be rebuilt.

This commit adds support for filtering the list of files in a directory
considered to be part of a package by using `git ls-files`. This git-specific
logic can be extended to other VCSs when cargo grows support for them.

Closes #379