Silence rename() in GoFmt

vim's rename() function will output messages for non-fatal conditions,
such as setting SELinux context. This will cause vim-go to annoyingly
display a "Press ENTER to continue" message in conjunction with
go_fmt_autosave = 1.

For example, if the original Go source file is located on a FUSE
filesystem that does not support `setxattr()`, and vim is compiled with
SELINUX_ENABLED, `rename()` will attempt to call `setfilecon()`, which
will fail and output an annoying error message.

Call stack:

```
 #0  mch_copy_sec (from_file=0xc2e3d0 "/tmp/vRUr7yU/7.go", to_file=0xc59910 "/fuse/test.go") at os_unix.c:2866
 fatih#1  0x00000000004e7dcf in vim_rename (from=0xc2e3d0 "/tmp/vRUr7yU/7.go", to=0xc59910 "/fuse/test.go") at fileio.c:6731
 fatih#2  0x0000000000480ab7 in f_rename (argvars=0x7ffec6fddac0, rettv=0x7ffec6fddd08) at evalfunc.c:9646
 fatih#3  0x0000000000474011 in call_internal_func (name=0xc51b00 "rename", argcount=2, argvars=0x7ffec6fddac0, rettv=0x7ffec6fddd08) at evalfunc.c:1110
 fatih#4  0x0000000000660d8e in call_func (funcname=0xc62d20 "rename", len=6, rettv=0x7ffec6fddd08, argcount_in=2, argvars_in=0x7ffec6fddac0, argv_func=0x0, firstline=12, lastline=12, doesrange=0x7ffec6fddcfc,
     evaluate=1, partial=0x0, selfdict_in=0x0) at userfunc.c:1507
 fatih#5  0x0000000000660690 in get_func_tv (name=0xc62d20 "rename", len=6, rettv=0x7ffec6fddd08, arg=0x7ffec6fddd38, firstline=12, lastline=12, doesrange=0x7ffec6fddcfc, evaluate=1, partial=0x0, selfdict=0x0)
     at userfunc.c:455
 fatih#6  0x0000000000666961 in ex_call (eap=0x7ffec6fddf38) at userfunc.c:3171
 fatih#7  0x00000000004af3c2 in do_one_cmd (cmdlinep=0x7ffec6fde6c8, sourcing=1, cstack=0x7ffec6fde210, fgetline=0x666be0 <get_func_line>, cookie=0xc48fd0) at ex_docmd.c:2537
 fatih#8  0x00000000004ac3d8 in do_cmdline (cmdline=0x0, fgetline=0x666be0 <get_func_line>, cookie=0xc48fd0, flags=7) at ex_docmd.c:1038
 fatih#9  0x0000000000662123 in call_user_func (fp=0xbfe220, argcount=2, argvars=0x7ffec6fdf0e0, rettv=0x7ffec6fdf328, firstline=12, lastline=12, selfdict=0x0) at userfunc.c:954
 fatih#10 0x0000000000660d01 in call_func (funcname=0xc59fb0 "go#fmt#update_file", len=18, rettv=0x7ffec6fdf328, argcount_in=2, argvars_in=0x7ffec6fdf0e0, argv_func=0x0, firstline=12, lastline=12,
     doesrange=0x7ffec6fdf31c, evaluate=1, partial=0x0, selfdict_in=0x0) at userfunc.c:1488
 fatih#11 0x0000000000660690 in get_func_tv (name=0xc59fb0 "go#fmt#update_file", len=18, rettv=0x7ffec6fdf328, arg=0x7ffec6fdf358, firstline=12, lastline=12, doesrange=0x7ffec6fdf31c, evaluate=1, partial=0x0,
     selfdict=0x0) at userfunc.c:455
 fatih#12 0x0000000000666961 in ex_call (eap=0x7ffec6fdf558) at userfunc.c:3171
 fatih#13 0x00000000004af3c2 in do_one_cmd (cmdlinep=0x7ffec6fdfce8, sourcing=1, cstack=0x7ffec6fdf830, fgetline=0x666be0 <get_func_line>, cookie=0xc61900) at ex_docmd.c:2537
 fatih#14 0x00000000004ac3d8 in do_cmdline (cmdline=0x0, fgetline=0x666be0 <get_func_line>, cookie=0xc61900, flags=7) at ex_docmd.c:1038
 fatih#15 0x0000000000662123 in call_user_func (fp=0xc59340, argcount=1, argvars=0x7ffec6fe0700, rettv=0x7ffec6fe0948, firstline=12, lastline=12, selfdict=0x0) at userfunc.c:954
 fatih#16 0x0000000000660d01 in call_func (funcname=0xc64a40 "go#fmt#Format", len=13, rettv=0x7ffec6fe0948, argcount_in=1, argvars_in=0x7ffec6fe0700, argv_func=0x0, firstline=12, lastline=12, doesrange=0x7ffec6fe093c,
     evaluate=1, partial=0x0, selfdict_in=0x0) at userfunc.c:1488
 fatih#17 0x0000000000660690 in get_func_tv (name=0xc64a40 "go#fmt#Format", len=13, rettv=0x7ffec6fe0948, arg=0x7ffec6fe0978, firstline=12, lastline=12, doesrange=0x7ffec6fe093c, evaluate=1, partial=0x0, selfdict=0x0)
     at userfunc.c:455
 fatih#18 0x0000000000666961 in ex_call (eap=0x7ffec6fe0b78) at userfunc.c:3171
 fatih#19 0x00000000004af3c2 in do_one_cmd (cmdlinep=0x7ffec6fe1308, sourcing=1, cstack=0x7ffec6fe0e50, fgetline=0x4cc2a0 <getexline>, cookie=0x0) at ex_docmd.c:2537
 fatih#20 0x00000000004ac3d8 in do_cmdline (cmdline=0xc65380 "call go#fmt#Format(-1)", fgetline=0x4cc2a0 <getexline>, cookie=0x0, flags=11) at ex_docmd.c:1038
 fatih#21 0x00000000004ba24b in do_ucmd (eap=0x7ffec6fe15d8) at ex_docmd.c:7018
 fatih#22 0x00000000004af390 in do_one_cmd (cmdlinep=0x7ffec6fe1d68, sourcing=0, cstack=0x7ffec6fe18b0, fgetline=0x4cc2a0 <getexline>, cookie=0x0) at ex_docmd.c:2528
 fatih#23 0x00000000004ac3d8 in do_cmdline (cmdline=0x0, fgetline=0x4cc2a0 <getexline>, cookie=0x0, flags=0) at ex_docmd.c:1038
 fatih#24 0x0000000000558c59 in nv_colon (cap=0x7ffec6fe1eb8) at normal.c:5388
 fatih#25 0x000000000054ec81 in normal_cmd (oap=0x7ffec6fe1f58, toplevel=1) at normal.c:1121
 fatih#26 0x00000000006bf4b9 in main_loop (cmdwin=0, noexmode=0) at main.c:1374
 fatih#27 0x00000000006bdf2b in vim_main2 () at main.c:920
 fatih#28 0x00000000006bb637 in main (argc=2, argv=0x7ffec6fe2168) at main.c:444
```

Fixes fatih#1081

autoload/go/fmt.vim

@@ -105,7 +105,7 @@ function! go#fmt#update_file(source, target)
     let original_fperm = getfperm(a:target)
   endif
 
-  call rename(a:source, a:target)
+  silent! call rename(a:source, a:target)
 
   " restore file permissions
   if exists("*setfperm") && original_fperm != ''