cache should contain on-disk (filtered) file size

The cache should contain the on-disk file size.  If we apply a filter, we should update the index's size.  Using `\n` -> `\r\n` conversion via `core.autocrlf=true` on a file containing only `\n`'s:

With core git:

    C:\Temp> git clone https://github.com/ethomson/crlf
    Cloning into 'crlf'...
    remote: Counting objects: 35, done.
    remote: Compressing objects: 100% (17/17), done.
    remote: Total 35 (delta 2), reused 35 (delta 2)
    Unpacking objects: 100% (35/35), done.
    C:\Temp> cd crlf
    C:\Temp\crlf> git status
    # On branch master
    nothing to commit, working directory clean
    C:\Temp\crlf> hexdump all-lf
      00000000  6c 66 0d 0a  6c 66 0d 0a  6c 66 0d 0a  6c 66 0d 0a  [lf.. lf.. lf.. lf..]
      00000001  6c 66 0d 0a                                        [lf..            ]
    C:\Temp\crlf> git ls-files --stage --debug all-lf
    100644 799770d1cff46753a57db7a066159b5610da6e3a 0       all-lf
      ctime: 1358454081:0
      mtime: 1358454081:0
      dev: 0        ino: 0
      uid: 0        gid: 0
      size: 20      flags: 0

With libgit2:

    C:\Temp> cgit2 clone https://github.com/ethomson/crlf crlf
      ....
    C:\Temp> cd crlf
    C:\Temp> git status all-lf
    # On branch master
    # Changes not staged for commit:
    #   (use "git add <file>..." to update what will be committed)
    #   (use "git checkout -- <file>..." to discard changes in working directory)
    #
    #       modified:   all-lf
    #
    no changes added to commit (use "git add" and/or "git commit -a")
    
    C:\Temp> hexdump all-lf
      00000000  6c 66 0d 0a  6c 66 0d 0a  6c 66 0d 0a  6c 66 0d 0a [lf..lf..lf..lf..]
      00000001  6c 66 0d 0a                                        [lf..            ]
    C:\Temp\crlf> git ls-files --stage --debug all-lf
    100644 799770d1cff46753a57db7a066159b5610da6e3a 0       all-lf
      ctime: 1358456764:0
      mtime: 1358456764:0
      dev: 0        ino: 0
      uid: 0        gid: 0
      size: 15      flags: 0