parsing.c: make commit buffer const

This will be required in order to incorporate the changes to commit
buffer handling in Git 2.0.2.

Signed-off-by: John Keeping <john@keeping.me.uk>
git: update to v2.0.3

This is slightly more involved than just bumping the version number
because it pulls in a change to convert the commit buffer to a slab,
removing the "buffer" field from "struct commit".  All sites that access
"commit->buffer" have been changed to use the new functions provided for
this purpose.

Signed-off-by: John Keeping <john@keeping.me.uk>
ui-stats.c: set parent pointer to NULL after freeing it

We do this everywhere else, so we should be doing it here as well.

Signed-off-by: John Keeping <john@keeping.me.uk>