Add support to git describe option --first-parent

What steps will reproduce the problem?

1. Go Settings/General/Dialogs1
2. Try to setup "Describe section"

What is the expected output? What do you see instead?

Check box to use or not --first-parent option of git describe command should be supported but not available

What version of TortoiseGit and Git are you using? On what operating system?

• TortoiseGit 2.6.0.0
• git version 2.10.2.windows.1

Please provide any additional information below.

It very helpfull option allow us to correct describe of commit after merging branches. Without this option incorrect tag used when describe(it taken from second parent and in some cases that is wrong. At least for us)