xopen: explicitly report creation failures

If the flags O_CREAT and O_EXCL are both given then open(2) is supposed
to create the file and error out if it already exists.  The error
message in that case looks like this:

	fatal: could not open 'foo' for writing: File exists

Without further context this is confusing: Why should the existence of
the file pose a problem?  Isn't that a requirement for writing to it?

Add a more specific error message for that case to tell the user that we
actually don't expect the file to preexist, so the example becomes:

	fatal: unable to create 'foo': File exists

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

wrapper.c

@@ -193,7 +193,9 @@ int xopen(const char *path, int oflag, ...)
 		if (errno == EINTR)
 			continue;
 
-		if ((oflag & O_RDWR) == O_RDWR)
+		if ((oflag & (O_CREAT | O_EXCL)) == (O_CREAT | O_EXCL))
+			die_errno(_("unable to create '%s'"), path);
+		else if ((oflag & O_RDWR) == O_RDWR)
 			die_errno(_("could not open '%s' for reading and writing"), path);
 		else if ((oflag & O_WRONLY) == O_WRONLY)
 			die_errno(_("could not open '%s' for writing"), path);