Don't write out a new .gitmodules on every submodule commit
Break setupIncremental calls into a separate loop

...since we'll need all the repositories present in order to restore
submodule references from the log file.
Introduce RepoIndex typedef for convenience
Pass the repo index to setupIncremental

We'll need it to restore submodule references
Write submodule list updates in fast-import progress messages
Read submodule list updates in setupIncremental
BUGFIX: boost is not a submodule of itself!

Fixes #21
Merge branch 'master' of github.com:ryppl/Boost2Git

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