Fix general example memory leaks

Use a shared buffer in calls of git_treebuilder_write to avoid heap contention

The changes to `git_submodule_update_option` are now landing in v.Next.

indexer: introduce `git_packfile_close`

It must be non-NULL to have a valid `git_indexer`.

Encapsulation!

Submodule optimization

Only try to `unlink` our temp file when we know that we didn't copy it
into its permanent location.

`git_submodule_status` is very slow, bottlenecked on
`git_repository_head_tree`, which it uses through `submodule_update_head`.  If
the user has requested submodule caching, assume that they want this status
cached too and skip it.

Signed-off-by: David Turner <dturner@twosigma.com>

Added `git_repository_submodule_cache_all` to initialze a cache of
submodules on the repository so that operations looking up N
submodules are O(N) and not O(N^2).  Added a
`git_repository_submodule_cache_clear` function to remove the cache.

Also optimized the function that loads all submodules as it was itself
O(N^2) w.r.t the number of submodules, having to loop through the
`.gitmodules` file once per submodule.  I changed it to process the
`.gitmodules` file once, into a map.

Signed-off-by: David Turner <dturner@twosigma.com>

Signed-off-by: David Turner <dturner@twosigma.com>

WinHTTP: support best auth mechanism

For username/password credentials, support NTLM or Basic (in that order
of priority).  Use the WinHTTP built-in authentication support for both,
and maintain a bitfield of the supported mechanisms from the response.

Fix handling of GIT_CHECKOUT_DISABLE_PATHSPEC_MATCH flag.

merge: set default rename threshold

giterr_set: consistent error messages

fix examples/network/clone.c: heap-buffer-overflow

security updates for v0.25

None of our crypto backends actually reject RC4 as a cipher so don't
test for it and instead keep it as something we'd like to do.

We must make sure that we're getting a certificate error from the
library so we know that we're testing the right thing.

Format of a length of string to the correct format is:%.*s

The Git protocol does not specify what should happen in the case
of an empty packet line (that is a packet line "0004"). We
currently indicate success, but do not return a packet in the
case where we hit an empty line. The smart protocol was not
prepared to handle such packets in all cases, though, resulting
in a `NULL` pointer dereference.

Fix the issue by returning an error instead. As such kind of
packets is not even specified by upstream, this is the right
thing to do.

Each packet line in the Git protocol is prefixed by a four-byte
length of how much data will follow, which we parse in
`git_pkt_parse_line`. The transmitted length can either be equal
to zero in case of a flush packet or has to be at least of length
four, as it also includes the encoded length itself. Not
checking this may result in a buffer overflow as we directly pass
the length to functions which accept a `size_t` length as
parameter.

Fix the issue by verifying that non-flush packets have at least a
length of `PKT_LEN_SIZE`.

Make sure that the callbacks do also get a 'valid' value of zero when
the certificate we're looking at is in valid and assert that within the
test.

Mention field addition in breaking API changes

I forgot that Windows chokes while trying to delete open files.