[StGIT PATCH 0/4] Rename detection, take 2

This series brings the ability to pass arbitrary flags to git-diff through StGIT commands. Eg:

    stg files -O -C
    stg diff -O '-C -w --color'
    stg mail -O --binary

The latter shows that the recently-added --binary is not needed any more, so I removed it.

While doing this, I realized that many git commands were spawned through shell commands instead of directly exec'ing them, and that was annyoing for consistency, so this is part of the same patch. As a bonus, it slightly improves performance.