Add git_diff_commit and last-changed examplerb/commit-modified-file

This adds a new diff API `git_diff_commit` which makes it easy to
generate a `git_diff` object that represents the changes in a given
commit.  It follows the core Git rules for considering changes in
a merge commit - i.e. if a file has an exact match in any parent
of the commit, then the file is considered unmodified by the merge.

This also adds a new example program "last-changed" which takes a
list of filenames and for each one displays the SHA of the last
commit that changed that file.

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