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>

builtin-apply.c

@@ -2416,8 +2416,7 @@ static void create_one_file(char *path, unsigned mode, const char *buf, unsigned
 		 * used to be.
 		 */
 		struct stat st;
-		errno = 0;
-		if (!lstat(path, &st) && S_ISDIR(st.st_mode) && !rmdir(path))
+		if (!lstat(path, &st) && (!S_ISDIR(st.st_mode) || !rmdir(path)))
 			errno = EEXIST;
 	}