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 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.
This introduces the functionality of submodule update in
'git_submodule_do_update'. The existing 'git_submodule_update' function is
renamed to 'git_submodule_update_strategy'. The 'git_submodule_update'
function now refers to functionality similar to `git submodule update`,
while `git_submodule_update_strategy` is used to get the configured value
of submodule.<name>.update.
