mingw32-git: Add html documentation

Fetch documentation from gitster/git-htmldocs and install it into the default htmldir. The patch remove-htmldir.patch" restores the default htmldir (share/doc/git-doc) which is in our case then /mingw/share/doc/git-doc. The previously used htmldir doc/git/html was very non-*nix-like.

The makefile target "html" is not usable for us as it depends on the not-available asciidoc/python programs.

The main reason I added the patch file was that I don't know a way to make a pull request for commits from different, git and mingwGitDevEnv-packages, repositories.

There are currently not all commands in "git help -a" listed. I suspect that this has to do with sschuberth/git@31e435d9e49372cb6ab31b6de530fbcc1ae8a533.
sschuberth/git/help.c/load_command_list(..) parses the files in exec_dir and generates from that the list of commands.

Try e. g
```
git help config
```
in mingwGitDevEnv (fails)  and msysgit (sucess).