You could set up an external tool that did something like the following:
Title: Commit with Selected Text as Message
Command: path\to\git.exe
Initial Directory: $(SolutionDir)
Arguments: commit --allow-empty -m "$(CurText)"
This will allow you to create an empty commit, with the selected text becoming the commit message. (Just bear in mind if you have any files staged, they will be committed as well)
I unfortunately do not have Visual Studio currently installed, so please take the above as a rough sketch, and my apologies if it doesn't work as is. But, I hope it is of use to you.
However, it is quite likely you can use Visual Studio's "External Tools" feature to be able to achieve this. (https://msdn.microsoft.com/en-us/library/76712d27.aspx)
You could set up an external tool that did something like the following:
Title: Commit with Selected Text as Message
Command: path\to\git.exe
Initial Directory: $(SolutionDir)
Arguments: commit --allow-empty -m "$(CurText)"
This will allow you to create an empty commit, with the selected text becoming the commit message. (Just bear in mind if you have any files staged, they will be committed as well)
I unfortunately do not have Visual Studio currently installed, so please take the above as a rough sketch, and my apologies if it doesn't work as is. But, I hope it is of use to you.