protocol: basic support for multi_ack_detailed

This fixes #1703.

This tells the server that we speak it, but we don't make use of its
extra information to determine if there's a better place to stop
negotiating.

In a somewhat-related change, reorder the capabilities so we ask for
them in the same order as git does.

Also take this opportunity to factor out a fairly-indented portion of
the negotiation logic.

Add support for thin packs

Test for failure, but don't test the error message

Rename the ssh credentials

Make reflog part of refdb

The "common.h" should be included before "config.h".

When building libgit2 for ia32 architecture on a x64 machine, including
"config.h" without a "common.h" would result the following error:
C:\Program Files\Microsoft SDKs\Windows\v7.1\include\winbase.h(2288): error C2373: 'InterlockedIncrement' : redefinition; different type modifiers [C:\cygwin\home\zcbenz\codes\git-utils\build\libgit2.vcxproj]
C:\Program Files\Microsoft SDKs\Windows\v7.1\include\winbase.h(2295): error C2373: 'InterlockedDecrement' : redefinition; different type modifiers [C:\cygwin\home\zcbenz\codes\git-utils\build\libgit2.vcxproj]
C:\Program Files\Microsoft SDKs\Windows\v7.1\include\winbase.h(2303): error C2373: 'InterlockedExchange' : redefinition; different type modifiers [C:\cygwin\home\zcbenz\codes\git-utils\build\libgit2.vcxproj]
C:\Program Files\Microsoft SDKs\Windows\v7.1\include\winbase.h(2314): error C2373: 'InterlockedExchangeAdd' : redefinition; different type modifiers [C:\cygwin\home\zcbenz\codes\git-utils\build\libgit2.vcxproj]

remove rpm spec file and readme

Fedora provides a rpm package for libgit2 -
https://apps.fedoraproject.org/packages/libgit2.

Allow -DWINHTTP=OFF to disable WinHTTP

Use two calls to set two headers

fix typos in README.md

It turns out that variables have function scope by default. Let's
really set -liconv and add a few libraries that were forgotten in
the previous commit.

We also need to special-case OSX, as they ship zlib but do not provide
a pkg-config file for it.

Be explicit about dependencies

transport: let the progress output return an error

There are any number of issues that can come up in the progress
callback, and we should let the user cancel at that point as well.

The user is unable to derive the number of deltas in the pack, as that
would require them to capture the stats exactly in the moment between
download and final processing, which is abstracted away in the fetch.

Capture these numbers for the user and expose them in the progress
struct. The clone and fetch examples now also present this information
to the user.

Show how many local objects were used to fix the thin pack in our fetch
example.

When linking statically, the including project needs to know what the
current library build depends on so they can link to it. Store this
information in the pkg-config file.

While here, remove claims that users need to link to zlib or libcrypto.

The names from libssh2 are somewhat obtuse for us. We can simplify the
usual key/passphrase credential's name, as well as make clearer what the
custom signature function is.

RFC: Proposed reworking of diff APIs

It seems that to implement these options, we just have to pass
the appropriate flags through to the libxdiff code taken from
core git.  So let's do it (and add a test).

Instead of having functions with so very many parameters to pass
hunk and line data, this takes the existing git_diff_hunk struct
and extends it with more hunk data, plus adds a git_diff_line.
Those structs are used to pass back hunk and line data instead of
the old APIs that took tons of parameters.

Some work that was previously only being done for git_diff_patch
creation (scanning the diff content for exact line counts) is now
done for all callbacks, but the performance difference should not
be noticable.

Allowed credential types should be a bitfield

While the base git_diff_delta structure always contains two files,
when we introduce conflict data, it will be helpful to have an
indicator when an additional file is involved.

license clarification + formatting all over

fixed typo `can can` -> `can`