Enhance rebase support on upstreams

[aiyengar2]

Support a workflow that makes it easier to be able to rebase existing patches on a chart to a new package.yaml upstream.

Add a Bash script to help interactively perform rebases on upstreams using git subtree and bash shell

Edit: See below.

[aiyengar2]

PR is already ready to go for this, we just need more testing for it.

[aiyengar2]

In favor of using the bash scripts, an implementation is described in #76 (comment) wherein we perform a weave merge in order to rebase to an upstream, which is 1) far better / more efficient than the existing solution of utilizing git subtrees and cherry-picking commits and 2) capable of supporting rebases outside of just Git-based packages