Don't recompile git deps so frequently

The previous logic for recompiling any dependency was almost entirely based on
the mtimes of the relevant input files. This isn't quite what's desired for git
and registry dependencies because the mtime could be fluctuating while the files
aren't changing. For example:

1. Project A checks out git repo C at revision C1
2. Project A builds, records mtimes of C
3. Project B checks out git repo C at revision C2
4. Project B builds, records new mtimes of C
5. Project A is rebuilt, rebuilding C b/c mtimes are different

In step 5 here C should not be rebuilt because the revision didn't actually
change.

This commit alters git/registry dependencies to completely bypass the --dep-info
emitted and only rely on the git/registry source to inform what the fingerprint
is. This is the revision/version, respectively, and should be all that's
necessary to track changes to the repo and trigger a rebuild.
auto merge of #548 : alexcrichton/cargo/frobbing-git-deps, r=brson

The previous logic for recompiling any dependency was almost entirely based on
the mtimes of the relevant input files. This isn't quite what's desired for git
and registry dependencies because the mtime could be fluctuating while the files
aren't changing. For example:

1. Project A checks out git repo C at revision C1
2. Project A builds, records mtimes of C
3. Project B checks out git repo C at revision C2
4. Project B builds, records new mtimes of C
5. Project A is rebuilt, rebuilding C b/c mtimes are different

In step 5 here C should not be rebuilt because the revision didn't actually
change.

This commit alters git/registry dependencies to completely bypass the --dep-info
emitted and only rely on the git/registry source to inform what the fingerprint
is. This is the revision/version, respectively, and should be all that's
necessary to track changes to the repo and trigger a rebuild.