Deal with empty and nonexsitent values in config

Returning NULL for the string when we haven't signaled an error
condition is counter-intuitive and causes unnecessary edge
cases. Return an empty string when asking for a string value for a
configuration variable such as '[section] var' to avoid these edge
cases.

If the distinction between no value and an empty value is needed, this
can be retrieved from the entry directly. As a side-effect, this
change stops the int parsing functions from segfaulting on such a
variable.

Update checkout with new strategies & behavior

'[section] variable' and '[section] variable =' behave differently
when parsed as booleans, so we need to store that distinction
internally.

We're already in the git_config namespace, there is no need to repeat
it.

Win32 CryptoAPI and CNG support for SHA1

Bump the builtin http-parser to v2.0

Fix memory leaks

repository: Refine repository_head() error report

The indexer handles this better than the fetch
logic does.

create FETCH_HEAD specially instead of as a ref file

Duplicate tree entries

Topic/explicit index errors

This fixes some various warnings that showed up in Travis and
a couple uses of uninitialized memory and one memory leak.

This fixes a number of warnings and problems with cross-platform
builds.  Among other things, it's not safe to name a member of a
structure "strcmp" because that may be #defined.

The `git_reset` API with the HARD option is still slightly
broken, but this test now does exercise the ability of the
command to revert modified files.

This is a major reworking of checkout strategy options.  The
checkout code is now sensitive to the contents of the HEAD tree
and the new options allow you to update the working tree so that
it will match the index content only when it previously matched
the contents of the HEAD.  This allows you to, for example, to
distinguish between removing files that are in the HEAD but not
in the index, vs just removing all untracked files.

Because of various corner cases that arise, etc., this required
some additional capabilities in rmdir and other utility functions.

This includes the beginnings of an implementation of code to read
a partial tree into the index based on a pathspec, but that is
not enabled because of the possibility of creating conflicting
index entries.