If on PowerShell v3 try this:
$out = git log ORIG_HEAD --% --no-merges --date=short --pretty="format:<tr><td>%h</td><td>%ad</td><td>%an</td><td>%s</td></tr>"
$out > test.txt
The --% puts PowerShell into a different parsing mode more suitable for native executables. See this blog post for more details.
If you're not using PowerShell v3, I suggest you use echoargs from the PowerShell Community Extensions to see the arguments as git.exe receives them from PowerShell e.g.:
PS> echoargs log ORIG_HEAD --no-merges --date=short --pretty="format:<tr><td>%h</td><td>%ad</td><td>%an</td><td>%s</td></tr>"
Arg 0 is <log>
Arg 1 is <ORIG_HEAD>
Arg 2 is <--no-merges>
Arg 3 is <--date=short>
Arg 4 is <--pretty=format:<tr><td>%h</td><td>%ad</td><td>%an</td><td>%s</td></tr>>
Command line:
"C:\Program Files (x86)\PowerShell Community Extensions\Pscx3\Pscx\Apps\EchoArgs.exe" log ORIG_HEAD --no-merges --date=short --pretty=format:<tr><td>%h</td><td>%ad</td><td>%an</td><td>%s</td></tr>
If you can see how PowerShell is passing the arguments to the exe you have a fighting chance to figure out how to massage the arguments which usually involves using extra quotes.