feat: add stg branch --reset command to soft reset the stack

[NonLogicalDev]

Introduce alternative to stg repair for more manual control over stack metadata repair process.

Example usage:

• Run stg branch --reset to force mark all patches in the stack as unapplied without changing HEAD.
• Manually reconcile the stack state by running stg push --merged or stg uncommit.

issue: #181

[NonLogicalDev]

Hello @jpgrayson, I have been using a private fork with this command for a few months now. It can come in handy in cases where stg repair either takes too long or you just expect it to get confused.

I think stg branch --reset + stg uncommit is probably the simplest and most extensible building block for building custom repair workflows.

[jpgrayson]

I don't fully understand the use case for this, but I'll accept the feature. I know this is something you've been thinking about for a long time. Very much appreciate the PR.

Also, the code is clean, thank you.

However, what I need is a couple tests that exercise this code path.

[NonLogicalDev]

Will add the tests and resubmit! Thank you @jpgrayson

As for the usecase: In my company we have lots of tools that very eagerly re-write local history.

Say I have a stack of changes:

GIT_HEAD_SHA: abc003, STG_EXPECTS: abc003
+ patch-1          # patch 1 changeset (abc001)
+ patch-2          # patch 2 changeset (abc002)
> patch-3          # patch 3 changeset (abc003)
- uapplied-patch-4 # patch-4 changeset (abc004)
- uapplied-patch-5 # patch-5 changeset (abc005)

I run bespoke-tool submit, and it goes ahead and re-writes the currently applied patches, and perhaps also pull from upstream:

++ [10 new commits from upstream got pulled in]
GIT_HEAD_SHA: abc017, STG_EXPECTS: abc003
+ patch-1          # patch 1 changeset rebased (abc015)
+ patch-2          # patch 2 changeset rebased (abc016)
> patch-3          # patch 3 changeset rebased (abc017)
- uapplied-patch-4 # patch-4 changeset (abc004)
- uapplied-patch-5 # patch-5 changeset (abc005)

There are currently two ways to recover:

• stg repair which will attempt to reconcile the mismatches patches
• git reset --hard $(stg id $(stg top)) to blow away the changes made by the rebase and return to the previous good state according to stg

I am introducing third more explicit way with this change:

• stg branch --reset -- trust git, and pretend all patches are now unapplied, and punt further reconcile to the user or a script