drush fserver-package: failed to get timestamp from git log command

This leads to an empty timestamp in .info file within created release node,
whille created for release node takes time() instead.

[...]
; Information added by fserver
core = "6.x"
datestamp = ""
project = "foobar"
project status url = "http://example.com/fserver"
version = "6.x-1.2"

Drush -vd output for drush -l example.com fserver-package foobar:

It seems that something went wrong at drush_shell_exec() 2 times,
one time for node created, another for .info´s datestamp.

[...]
Calling drush_shell_exec(git 	/tmp/fserver_git_foobar_1335100265/.git log -1 --pretty=format:%ct 6.x-1.2) [2.73 sec, 16.5 MB]           [debug]
sprintf(): Too few arguments exec.inc:114 [2.73 sec, 16.5 MB]                                                                                     [warning]
Executing: 
Undefined offset: 0 fserver.drush.inc:373 [2.73 sec, 16.5 MB]                                                                                        [notice]
Calling drush_shell_exec(git --git-dir=/tmp/fserver_git_foobar_1335100265/.git tag -l 6.x-1.2 -n 1000) [2.73 sec, 16.5 MB]                       [debug]
Executing: git --git-dir=/tmp/fserver_git_foobar_1335100265/.git tag -l 6.x-1.2 -n 1000
  6.x-1.2         Forgot title substitution: %1 -> Release for %1.
Calling drush_shell_exec(git --git-dir=/tmp/fserver_git_foobar_1335100265/.git log -1 --pretty=format:%ct 6.x-1.2) [2.73 sec, 16.5 MB]           [debug]
sprintf(): Too few arguments exec.inc:114 [2.73 sec, 16.5 MB]                                                                                     [warning]
Executing: 
Undefined offset: 0 fserver.drush.inc:373 [2.73 sec, 16.5 MB] 
[...]

The single command invoke from cli works within my workingcopy.

$ git log -1 --pretty=format:%ct 6.x-1.2
1334676452
$ git --git-dir=.git log -1 --pretty=format:%ct 6.x-1.2
1334676452

BTW:

• drush version 5.1
• git version 1.7.5.4
• PHP 5.3.6-13ubuntu3.6 with Suhosin-Patch (cli)

Anyone the same behaviour?