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>

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