shortcut exprs and cmds for git

In Git, everything is possible. But for some common commands I wrote these shortcut scripts. Some of them are like helper expressions to use inside another Git cmd line.

Most of them had to be implemented on the Plumbing level (which means, a little bit of hacking), but provide features useful for me such as:

git heads-for-merge -- what does your FETCH_HEAD say about all the heads that have been fetched for merge?

git the-empty-tree -- the ID to use if you need to refer to the empty tree in your (tree-merging) commands

git mread-and-commit -- read in multiple refs, merge the trees, and then make a merge commit, without touching your index or the working dir (useful for managing imported histories)

git merge-without-working -- the same idea -- advance your HEAD by merging

And more...