When you are in command line interface (assume the PATH is good), just use:
TortoiseGitProc /command:log
otherwise,
TortoiseGitProc /command:log /path:"the\path\you\want\to\open\log"
cf. TortoiseGit Manual - Appendix D. Automating TortoiseGit
For getting the exact command argument, go to TortoiseGit -> Settings -> Advanced tree node -> set Option "Debug" value to true.
Then open the log via normal way, you will get extra message dialog. Something like:

Then use Ctrl + C to copy the whole message, and paste it to somewhere, ex: notepad, and reuse the command line.
(BTW, you can easily ignore the /hwnd argument.)