Refactoring of C function git2r_commit_create

* src/git2r_commit.c (git2r_commit_create): Split function into
  several functions.

* src/git2r_commit.c (git2r_any_changes_in_index): Add function
  to check for changes in index.

* src/git2r_commit.c (git2r_parents_lookup): Add function to
  create and populate vector of commits.

* src/git2r_commit.c (git2r_parents_free): Add function to close
  the commits in parents and free memory of parents.

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>