Fix Issue #2003 and Update Doc

The first commit(Update libgit2) is NOT related to [Issue #2003](https://code.google.com/p/tortoisegit/issues/detail?id=2003).
I just want to update libgit2 first, because it fixes mixed EOL problem and makes the mixed EOL behavior is identical with official git.
(There is a [commit of libgit2](https://github.com/libgit2/libgit2/commit/d412165f948b77bf548234fd46fb0743f6afc3ed) fixes further EOL problem, but maybe update it later. It is related to Git for Windows 1.9.4)

The rest is trying to fix [Issue #2003](https://code.google.com/p/tortoisegit/issues/detail?id=2003) via 
 1. extending the GetOneFile()  to support blob hash as RefName
 2. and using new command "getonefile" to get file with correct EOL.


Oh~ I am happy to ignore it. ^_^
(I guess @lznuaa had tested it, when tried to fix [issue 542](https://code.google.com/p/tortoisegit/issues/detail?id=542). So he introduced the GetOneFile())

I am trying to implement the gitdll part, but I am not familiar with it.
Currently, 
I try to extend it inner git_checkout_file() to support blob
I have 2 way:
 1. try to fill the cache_entry var with right value
 2. read tree recursively to match blob and get the cache_entry

all of them is try to get right value of cache_entry
(blob only have the content, no file name, no mode...)
Then, I can re-use write_entry().
And...
The first way: I re-use file name from "path" parameter, all I need is file mode. mmm... trying to get it currently.
(I give the value 0x81a4(mode "10064") to test it.)
The second way: my plan B, don't have any code yet.
(I don't like plan B. the blob is so close, but it has to run a cycle to get it.)
Do I go the right direction?

Do you have a better idea to implement this part?
or give me some guide?
or wait for me? (maybe for few weeks)
for me, it is a exercise and a chance to study code.
(few weeks? yap, I guess I need few weeks to fight with it. :-) )

So if you can't wait, feel free to fix it. ^_^
(I can take others as exercise.)

**Thank you all. ^________^**