Checkout dry-run

In libgit2 nomenclature, when we need to verb a direct object, we name
a function `git_directobject_verb`.  Thus, if we need to init an options
structure named `git_foo_options`, then the name of the function that
does that should be `git_foo_options_init`.

The previous names of `git_foo_init_options` is close - it _sounds_ as
if it's initializing the options of a `foo`, but in fact
`git_foo_options` is its own noun that should be respected.

Deprecate the old names; they'll now call directly to the new ones.

Move to the `git_error` name in error-related functions, deprecating the
`giterr` functions.  This means, for example, that `giterr_last` is now
`git_error_last`.  The old names are retained for compatibility.

This only updates the public API; internal API and function usage
remains unchanged.