git-p4: Fix one-liner in p4_write_pipe function.

The function built a p4 command string via the p4_build_cmd function, but
ignored the result.
Signed-off-by: NTor Arvid Lund <torarvid@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/fast-import/git-p4

@@ -76,7 +76,7 @@ def write_pipe(c, str):
 
 def p4_write_pipe(c, str):
     real_cmd = p4_build_cmd(c)
-    return write_pipe(c, str)
+    return write_pipe(real_cmd, str)
 
 def read_pipe(c, ignore_error=False):
     if verbose: