By constructing an arg vector manually, we no longer need to quote arguments
Mandate that args must be passed when building a command
Now you need to provide an args array when building a command.
There are a handful of places where we need to deal with a string,
such as with user-defined custom commands, and for those we now require
that at the callsite they use str.ToArgv to do that. I don't want
to provide a method out of the box for it because I want to discourage its
use.
For some reason we were invoking a command through a shell when amending a
commit, and I don't believe we needed to do that as there was nothing user-
supplied about the command. So I've switched to using a regular command out-
side the shell there
The only exception is when moving a custom patch for an entire commit to an
earlier commit; in this case the source commit becomes empty, but we want to
keep it, mainly for consistency with moving the patch to a later commit, which
behaves the same.
In all other cases where we rebase, it's confusing when empty commits are kept;
the most common example is rebasing a branch onto master, where master already
contains some of the commits of our branch. In this case we simply want to drop
these.
I noticed that `$GIT_SEQUENCE_EDITOR` is overridden in `PrepareInteractiveRebaseCommand`
but not in `runSkipEditorCommand`.
Before this change, some commands such as `SquashAllAboveFixupCommits`
would not work when a different sequence editor, e.g.
[git-interactive-rebase-tool](https://github.com/MitMaro/git-interactive-rebase-tool)
is configured.
