Remove unneeded includes
Clean up libgit2 allocated memory
Fix incorrect comment

Revwalker begins by comparing the first commit to the second
commit - effectively obtaining the changes from commit #2.
The first diff should be between a NULL empty tree and the
first commit.
Get number of hunks in a commit

Right now, this can't get the number of hunks from a NULL patch - which
stems from trying to do a git_patch_from_diff when the diff has no deltas.