How can I move all git content one-level up in the folder hierarchy?

Another variant of Sumeet's answer - in the repository directory above "webapp" execute following command:

git mv webapp/* ./ -k

-k - Skip move or rename actions which would lead to an error condition, otherwise you get:

fatal: not under version control, source=webapp/somefile, destination=somefile

The right way to do this is:

git mv repo.git/webapp/* repo.git/.
git rm repo.git/webapp
git add * 
git commit -m "Folders moved out of webapp directory :-)"

I was able to get it to work simply by doing this from the destination folder:

git mv webapp/* .

In Windows shell the * will not work (fails with an error Bad source), but it will work in Windows if you use the Git Bash shell, which expands the * wildcard.

If you are using Powershell, you can also do:

git mv (gci webapp/*) .

or

git mv (get-childItem webapp/*) .