Add support for lowercase proxy environment variables

dirname with DOS prefixes

Flag options in git_stash_apply and git_stash_pop as being optional

rename detection: don't try to detect submodule renames

Test that shows that submodules are incorrectly considered in renames,
and `git_merge_trees` will fail to lookup the submodule as a blob.

The options parameter in both git_stash_apply and git_stash_pop can
be NULL. They should be flagged as such in the documentation.

Fix digest credentials for proxy in windows

curl supports HTTPS_PROXY in addition to https_proxy (and their http counterparts). This change ensures parity with curl's behavior.

As of recently, we failed to correctly discover repositories at a
Win32 system root. Instead of aborting the upwards-traversal of
the file system, we were looping infinitely when traversal
started at either a Win32 drive prefix ("C:/") or a network path
("//somehost").

The issue has been fixed, so add a test to catch regressions.

When calling `git_path_dirname_r` on a Win32 prefix, e.g. a drive
or network share prefix, we always want to return the trailing
'/'. This does not work currently when passing in a path like
'C:', where the '/' would not be appended correctly.

Fix this by appending a '/' if we try to normalize a Win32 prefix
and there is no trailing '/'.

Getting the dirname of a filesystem root should return the filesystem
root itself. E.g. the dirname of "/" is always "/". On Windows, we
emulate this behavior and as such, we should return e.g. "C:/" if
calling dirname on "C:/". But we currently fail to do so and instead
return ".", as we do not check if we actually have a Windows prefix
before stripping off the last directory component.

Fix this by calling out to `win32_prefix_length` immediately after
stripping trailing slashes, returning early if we have a prefix.

Extract code which determines if a path is at a Windows system's root.
This incluses drive prefixes (e.g. "C:\") as well as network computer
names (e.g. "//computername/").

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