Implement `git lfs clone`

Hi,

I found a way to make Git LFS faster up to a factor of 100x in
repositories with a large number of Git LFS files. I am looking
for comments if my approach would be acceptable by the Git community.

## What is Git LFS?
Git LFS [1] is an extension to Git that handles large files for Git
repositories. The project gained quite some momentum as almost all major
Git hosting services support it (GitHub [1], Atlassian Bitbucket [2],
GitLab [4]).

## What is the problem with Git LFS?
Git LFS is an application that is executed via Git clean/smudge filter.
The process invocation of these filters requires noticeable time (especially
on Windows) even if the Git LFS executable only accesses its local cache.

Based on my previous findings [5] Steve Streeting (@sinbad) improved the
clone times of Git LFS repositories with a lot of files by a factor of 10
or more [6][7].

Unfortunately that fix helps only with cloning. Any local Git operation
that invokes the clean/smudge filter (e.g. switching branches) is still
slow. Even on the Git mailing list a user reported that issue [8].

## Proposed solution
Git LFS caches its objects under .git/lfs/objects. Most of the time Git
LFS objects are already available in the cache (e.g. if you switch branches
back and forth). I implemented these "cache hits" natively in Git.
Please note that this implementation is just a quick and dirty proof of
concept. If the Git community agrees that this kind of approach would be
acceptable then I will start to work on a proper patch series with cross
platform support and unit tests.

## Performance tests
I executed both test runs on a 2,5 GHz Intel Core i7 with SSD and OS X.
A test run is the consecutive execution of four Git commands:
 1. clone the repo
 2. checkout to the "removed-files" branch
 3. timed: checkout the "master" branch
 4. timed: checkout "removed-files" branch

Test command:
set -x; git lfs clone https://github.com/larsxschneider/lfstest-manyfiles.git repo; cd repo; git checkout removed-files; time git checkout master; time git checkout removed-files

I compiled Git with the following flags:
NO_GETTEXT=YesPlease NEEDS_SSL_WITH_CRYPTO=YesPlease make -j 8 CFLAGS="-I/usr/local/opt/openssl/include" LDFLAGS="-L/usr/local/opt/openssl/lib"

### TEST RUN A -- Default Git 2.9 (ab7797d) and Git LFS 1.2.1
+ git lfs clone https://github.com/larsxschneider/lfstest-manyfiles.git repo
Cloning into 'repo'...
warning: templates not found /Users/lars/share/git-core/templates
remote: Counting objects: 15012, done.
remote: Total 15012 (delta 0), reused 0 (delta 0), pack-reused 15012
Receiving objects: 100% (15012/15012), 2.02 MiB | 1.77 MiB/s, done.
Checking connectivity... done.
Checking out files: 100% (15001/15001), done.
Git LFS: (15000 of 15000 files) 0 B / 77.04 KB
+ cd repo
+ git checkout removed-files
Branch removed-files set up to track remote branch removed-files from origin.
Switched to a new branch 'removed-files'
+ git checkout master
Checking out files: 100% (12000/12000), done.
Switched to branch 'master'
Your branch is up-to-date with 'origin/master'.

real    6m2.979s
user    2m39.066s
sys 2m41.610s
+ git checkout removed-files
Switched to branch 'removed-files'
Your branch is up-to-date with 'origin/removed-files'.

real    0m1.310s
user    0m0.385s
sys 0m0.881s

### TEST RUN B -- Default Git 2.9 with native LFS cache and Git LFS 1.2.1
https://github.com/larsxschneider/git/tree/lfs-cache
+ git lfs clone https://github.com/larsxschneider/lfstest-manyfiles.git repo
Cloning into 'repo'...
warning: templates not found /Users/lars/share/git-core/templates
remote: Counting objects: 15012, done.
remote: Total 15012 (delta 0), reused 0 (delta 0), pack-reused 15012
Receiving objects: 100% (15012/15012), 2.02 MiB | 1.44 MiB/s, done.
Checking connectivity... done.
Git LFS: (15001 of 15000 files) 0 B / 77.04 KB
+ cd repo
+ git checkout removed-files
Branch removed-files set up to track remote branch removed-files from origin.
Switched to a new branch 'removed-files'
+ git checkout master
Checking out files: 100% (12000/12000), done.
Switched to branch 'master'
Your branch is up-to-date with 'origin/master'.

real    0m2.267s
user    0m0.295s
sys 0m1.948s
+ git checkout removed-files
Switched to branch 'removed-files'
Your branch is up-to-date with 'origin/removed-files'.

real    0m0.715s
user    0m0.072s
sys 0m0.672s

### Results
Default Git:                      6m2.979s + 0m1.310s = 364s
Git with native LFS cache access: 0m2.267s + 0m0.715s = 4s

The native cache solution is almost 100x faster when switching branches
on my local machine with a test repository containing 15,000 Git LFS files.
Based on my previous experience with Git LFS clone I expect even more
dramatic results on Windows.

Thanks,
Lars

[1] https://git-lfs.github.com/
[2] https://github.com/blog/1986-announcing-git-large-file-storage-lfs
[3] http://blogs.atlassian.com/2016/02/git-lfs-for-designers-game-developers-architects/
[4] https://about.gitlab.com/2015/11/23/announcing-git-lfs-support-in-gitlab/
[5] git-lfs/git-lfs#931 (comment)
[6] git-lfs/git-lfs#988
[7] https://developer.atlassian.com/blog/2016/04/git-lfs-12-clone-faster/
[8] http://article.gmane.org/gmane.comp.version-control.git/297809