.gitattributes export-subst doesn't work correctly

Summary

.gitattributes export-subst are not being handled at all by git-export-all (upstream issue https://github.com/Kentzo/git-archive-all/issues/81). No progress on this is evident, and the git-export-all author's approach of having to recreate all git features is ultimately doomed to suffer more of these sorts of issues in the future.

As a workaround, we should at least allow use of plain git exports when the package doesn't contain any submodules, as probably applies to most packages, and especially to the ones I know are affected (the NodeCore ones).

Steps to reproduce

1. Create a release for NodeCore or NodeCore Alpha (or look at any of the releases already on CDB).
2. Run the game and open up the "inventory" screen (about dialog).

Expected:

The version number of the release is listed. These are calculated from a version.lua file that has git export substitutions in it to access the commit metadata at export time.

Observed:

The version is listed as "development/contentdb version" because the substitutions weren't performed.

Start by examining how git-export-all creates release archives and compare that flow with plain git exports for packages without submodules. Reproduce the issue with a NodeCore or NodeCore Alpha release and inspect version.lua, where git export substitutions are expected. Done means affected releases show their commit-derived version instead of "development/contentdb version".