Fix compile error on Visual Studio

Introduce GIT_MERGE_CONFIG_* for merge.ff settings

git_merge_analysis will now return GIT_MERGE_CONFIG_NO_FASTFORWARD
when merge.ff=false and GIT_MERGE_CONFIG_FASTFORWARD_ONLY when
merge.ff=true

Fixed miscellaneous documentation errors.

We go through the different backends in order, so it's not an error if
at least one of the backends has the data we want.

We have too many places where we repeat free code, so when adding the
new free to the generic code, it didn't take for the local transport.

While there, fix a C99-ism that sneaked through.

Add support for the symref extension

Make core.safecrlf not generate an error on LF-ending platforms

Include windows.h on win32 for Sleep

smart: send a flush when we disconnect

Since we no longer need to push data to callbacks, there's no need for
this truct.

Let's use the remote's default branch guessing instead of reinventing
one ourselves with callbacks.

If the remote supports the symref protocol extension, then we return
that, otherwise we guess with git's rules.

When using the local transport, we always have the symbolic information
available, so fill it.

Add a symref_target field to git_remote_head to expose the symref
mappings to the user.

The protocol has a capability which allows the server to tell us which
refs are symrefs, so we can e.g. know which is the default branch.

This capability is different from the ones we already support, as it's
not setting a flag to true, but requires us to store a list of
refspec-formatted mappings.

This commit does not yet expose the information in the reference
listing.

Minor fix for cmn/clone-into-mirror.

A recently added check might skip initialization of old_fetchhead and go
directly to cleanup. So, destruct in the opposite order of construction.

The git server wants to hear a flush from us when we disconnect,
particularly when we want to perform a fetch but are up to date.

If you enabled core.safecrlf on an LF-ending platform, we would
error even for files with all LFs.  We should only be warning on
irreversible mappings, I think.

Restore attributions for fnmatch

Allow mirror-clone via `git_clone_into()`

We set up the current branch after we fetch from the remote. This means
that the user's refspec may have already created this reference. It is
therefore not an error if we cannot create the branch because it already
exists.

This allows for the user to replicate git-clone's --mirror option.

Instead of changing the user-provided remote, duplicate it so we can add
the extra refspec without having to worry about unsetting it before
returning.

Show a failure to perform a mirror-clone from a repository, both local
and remote.

message: don't assume the comment char

WIP: Windows fixes

Fix warning on uninitialized variable.