Rename file to `git-meld` and add man page for git to automatically use it

[mechanical-snail]

If you rename the main executable from git-meld.pl to git-meld, then Git will automatically use it when you type git meld. This simplifies installation, since then you can just drop the script anywhere on the PATH, rather than having to define an alias.

git help meld and git meld --help will also automatically run man git-meld, which will work as expected if you create that man page.

[albfan]

This is my default approach for extending git commands. I will implemented it and send PR.