Use the new object_type enumeration names within the codebase.

Signed-off-by: Sven Strickroth <email@cs-ware.de>

If the index contains stat data for a modified file, and the file is
not racily dirty, and there exists an untracked working tree directory
alphabetically after that file, and there are no other changes to the
repo, then git_stash_save would fail. It would confuse the untracked
working tree directory for the modified file, because they have the
same sha: zero.  The wt directory has a sha of zero because it's a
directory, and the file would have a zero sha because we wouldn't read
the file -- we would just know that it doesn't match the index.  To
fix this confusion, we simply check mode as well as SHA.

Ensure that when a file is added in the index and subsequently
modified in the working directory, the stashed working directory
tree contains the actual working directory contents.

stash: correctly stash wd modified/index deleted

We want to use the "checkout: moving from ..." message in order to let
git know when a change of branch has happened. Make the convenience
functions for this goal write this message.

The signature for the reflog is not something which changes
dynamically. Almost all uses will be NULL, since we want for the
repository's default identity to be used, making it noise.

In order to allow for changing the identity, we instead provide
git_repository_set_ident() and git_repository_ident() which allow a user
to override the choice of signature.