Fix a few recent warnings

Clang analyzer run

Extend packfile in increments of page_size.

Fixups requested in #3912.

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

Otherwise we'll NULL-dereference in git_attr_cache__init

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