refdb: use the same id for old and new when renaming a reference

When we rename a reference, we want the old and new ids to be the same
one (as we did not change it). The normal code path looks up the old id
from the current value of the brtanch, but by the time we look it up, it
does not exist anymore and thus we write a zero id.

Pass the old id explicitly instead.

Filter relative paths

branch: fix generated reflog message upon creation

Test that filter_list_apply_to_file works and can accept repo-relative
paths.

Fix crash in git_clone on extremely large repos

Free TLS data on thread exit (win32)

Clear the error message on git_libgit2_shutdown for all versions of
the library (no threads and Win32 threads).  Drop the giterr_clear
in clar, as that shouldn't be necessary.

Set up git_trace in clar test suite.

diff_tform: don't compare empty hashsig_heaps

When comparing seemingly blank files, take whitespace options into
account.

Don't try to compare two empty hashsig_heaps.

http: enforce the credential types

Borrowing configuration entries

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.

The user may decide to return any type of credential, including ones we
did not say we support. Add a check to make sure the user returned an
object of the right type and error out if not.

Fix build on mingw (master branch)

stash: correctly stash wd modified/index deleted

Remove the signature from ref-modifying functions

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.

We always use "update by push".

This namespace is about behaving like git's branch command, so let's do
exactly that instead of taking a reflog message.

This override is still available via the reference namespace.

This function is meant to simulate what git does in the reset command,
so we should include the reflog message in that.

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.

describe example: function to add commits to opts