Silent failure to add Windows-style paths in Cygwin Git

[Adam Dinwoodie <adam@dinwoodie.org>]

If I attempt to `git add` an extant file specified using a Windows-style
path on Cygwin Git, this doesn't add the file, and produces no error
message:

    $ pwd  # As seen by Cygwin
    /cygdrive/c/tmp

    $ cygpath -aw .  # As seen by Windows
    C:\tmp

    $ git init
    Initialized empty Git repository in /cygdrive/c/tmp/.git/

    $ git add 'c:\tmp\file' || echo non-zero exit code  # Errors out as expected
    fatal: pathspec 'c:\tmp\file' did not match any files
    non-zero exit code

    $ touch file

    $ git add 'c:\tmp\file' || echo non-zero exit code  # No error this time...

    $ git status  # ...even though the file didn't get added
    On branch master

    Initial commit

    Untracked files:
      (use "git add <file>..." to include in what will be committed)

            file

    nothing added to commit but untracked files present (use "git add" to track)

I wouldn't expect adding the file to actually succeed, but I would
expect it to either succeed or produce an error, rather than silently
failing.

Experimentation shows I get the same behaviour for 'c:\tmp\file',
'c:/tmp/file' and 'subdir\file'.  I'm seeing this on v2.8.0; the
downstream report says the same behaviour occurs on v2.7.4[0], and I've
also seen what appears to be the same behaviour on a v2.0.5 build I
produced to check.

Adam

[0]: https://cygwin.com/ml/cygwin/2016-04/msg00474.html