[WIP/RFC] push: report the update plan to the caller

In checkout.c and filter.c we were casting a sub struct
to a parent struct which breaks the strict aliasing rules
in C. However we can use .parent or .base to access the
parent struct to avoid the build warnings.

In remote.c the local variable error was not initialized
or updated in some cases. For unintialized error a build
warning will be generated. So always keep error variable
up-to-date.

It can be useful for the caller to know which update commands will be
sent to the server before the packfile is pushed up. git does this via
the pre-push hook.

We don't have hooks, but as it adds introspection into what is
happening, we can add a callback which performs the same function.

Update tips fixes

Otherwise, bailing out early when ls_to_vector() fails accesses
uninitialized memory.

This changes the get_entry() method to return a refcounted version of
the config entry, which you have to free when you're done.

This allows us to avoid freeing the memory in which the entry is stored
on a refresh, which may happen at any time for a live config.

For this reason, get_string() has been forbidden on live configs and a
new function get_string_buf() has been added, which stores the string in
a git_buf which the user then owns.

The functions which parse the string value takea advantage of the
borrowing to parse safely and then release the entry.

We always use "update by push".

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.

Introduce some helper macros to test integer overflow from arithmetic
and set error message appropriately.