Add uncommit command

[zombiezen]

It would be nice to replicate the Mercurial uncommit command. Synopsis:

gg uncommit [FILE [...]]

uncommit part or all of a local commit

Running gg uncommit without any arguments should behave like git reset --soft HEAD~.

Running gg uncommit with arguments should do something like:

# Update index with previous state of files, but don't modify the working tree.
git reset -q HEAD~ -- FILE1 FILE2 ...
# Now roll back the HEAD ref.
git reset --soft HEAD~

Tricky part will be to ensure this works on the first commit as well: this shouldn't introduce any bugs like #30.

/cc @ianthehat, who requested this feature.

[zombiezen]

Correction: in the second case, it should amend the top commit to point to old content.

Oddly enough, I think the easiest way to implement this might be to read the HEAD tree, rewrite the content to point to the blobs for the given files at HEAD~, use git-commit-tree to create the new commit, then git reset --soft NEW_COMMIT_HASH. For merge commits, this would only work if the content of the blob was the same in all parents (it should error otherwise).