mingw: let `mingw_lstat()` error early upon problems with reparse points

mingw: let `mingw_lstat()` error early upon problems with reparse points

When obtaining lstat information for reparse points, we need to call
`FindFirstFile()` in addition to `GetFileInformationEx()` to obtain
the type of the reparse point (symlink, mount point etc.). However,
currently there is no error handling whatsoever if `FindFirstFile()`
fails.

Call `FindFirstFile()` before modifying the `stat *buf` output parameter
and error out if the call fails.

Note: The `FindFirstFile()` return value includes all the data
that we get from `GetFileAttributesEx()`, so we could replace
`GetFileAttributesEx()` with `FindFirstFile()`. We don't do that because
`GetFileAttributesEx()` is about twice as fast for single files. I.e.
we only pay the extra cost of calling `FindFirstFile()` in the rare case
that we encounter a reparse point.

Please also note that the indentation the remaining reparse point
code changed, and hence the best way to look at this diff is with
`--color-moved -w`. That code was _not_ moved because a subsequent
commit will move it to an altogether different function, anyway.

Signed-off-by: Karsten Blees <karsten.blees@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>