This is something a bit like git-add -i, except not depending on perl,
and partially crossed with git-commit.

Usage: $0 [file [file ...]]

Runs git diff with the same arguments.  Any non-text diffs, new files,
or deleted files cause an immediate error out.  Text diffs are
accumulated; each diff hunk is remembered separately.  The user
interface allows the user to, repeatedly, choose from:

- Quit.
- Choose a diff hunk to be included.
- Commit chosen hunks.

Committing works by constructing a git-fast-import pack for the new
commit, then running git-commit --amend to let the user supply (edit,
really) the commit message.  The major side effect of this is that
`aborting' the commit by giving an empty message does not back out the
whole thing, only the editing of the message; there is still a commit
created with an empty commit message.