global: use release_commit_memory()
Instead of calling two separate Git functions to free memory associated
with a commit object, use Git's wrapper which does this. This also
counts as a potential future bug fix since release_commit_memory() also
resets the parsed state of the commit, meaning any attempt to use it in
the future will correctly fill out the fields again.
release_commit_memory() does not set parents to zero, so keep that for
additional safety in case CGit checks this without calling
parse_commit() again.
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>