Fix overwriting of files when applying contextually independent diffs

Fix overwriting of files when applying contextually independent diffs

Noticed by applying two diffs of different contexts to the same file.

The check for existence of a file was wrong: the test assumed it was
a directory and reset the errno (twice: directly and by calling
lstat). So if an entry existed and was _not_ a directory no attempt
was made to rename into it, because the errno (expected by renaming
code) was already reset to 0. This resulted in error:

    fatal: unable to write file file mode 100644

For Linux, removing "errno = 0" is enough, as lstat wont modify errno
if it was successful. The behavior should not be depended upon,
though, so modify the "if" as well.

The test simulates this situation.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>