Fix digest credentials for proxy in windows

Flag checkout_opts in git_reset as optional

The check_outs argument can be NULL and should be flagged accordingly
in the header file.

Flag given_opts in git_revert as optional

The given_opts argument can actually be NULL and thus should be
flagged accordingly in the header file.

Vector reverse overflow

The code reversing a vector initially determines the rear-pointer by
simply subtracting 1 from the vector's length. Obviously, this fails if
the vector is empty, in which case we have an integer overflow.

Fix the issue by returning early if the vector is empty.

Update docs for git_oid_fromstrn and p

Fix uninitialized variable warning

Fix the following warning emitted by clang:
[ 16%] Building C object CMakeFiles/libgit2_clar.dir/src/submodule.c.o
/Users/mplough/devel/external/libgit2/src/submodule.c:408:6: warning: variable 'i' is used uninitialized whenever 'if' condition is true
      [-Wsometimes-uninitialized]
        if ((error = load_submodule_names(names, cfg)))
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/mplough/devel/external/libgit2/src/submodule.c:448:20: note: uninitialized use occurs here
        git_iterator_free(i);
                          ^
/Users/mplough/devel/external/libgit2/src/submodule.c:408:2: note: remove the 'if' if its condition is always false
        if ((error = load_submodule_names(names, cfg)))
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/mplough/devel/external/libgit2/src/submodule.c:404:17: note: initialize the variable 'i' to silence this warning
        git_iterator *i;
                       ^
                        = NULL
1 warning generated.

WIP: some coverity & compiler warning fixes

WinHTTP: set proper error messages when SSL fails

If `attr_cache_lookup_entry` fails to find the given file, make sure
that we do not try to free the given file.

When we fail to load submodules, don't free the list; it is later freed
unconditionally.

Fix a few recent warnings

Clang analyzer run

Extend packfile in increments of page_size.

Fixups requested in #3912.

Set up a WinHTTP status callback; inspect the WinHTTP status for
WINHTTP_CALLBACK_STATUS_SECURE_FAILURE, and convert the status code
to a useful message for callers.

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>