Bug: Pathspec parsing on Windows fails when branch specified

[Eris Belew <erisrenee@gmail.com>]

System: Windows Server 2008 R2
Git: git version 1.8.4.msysgit.0
Shell: Powershell V3 (No third-party modules loaded)

Summary:
  When specifying a pathspec including a branch/commit, path separator
characters are not translated. Since tab-completion in windows shells (ex:
CMD, PowerShell, not unix-style shells running on windows) uses the windows
path separator, and other git commands work fine with the windows path
separator, the expected behavior would be to translate for me.

Reproduction:
    git diff BRANCH:path\to\file path\to\file

Result:
    fatal: Path 'path\to\file' does not exist in 'BRANCH'

Expected:
    Normal diff operation

Workaround:
    Manually convert pathspec. Examples of working command:
        git diff BRANCH:path/to/file path\to\otherfile
        git diff BRANCH:path/to/file path/to/otherfile


Thanks,

Eris