helm: requirements.lock does not get updated on helm dep up

if there is no change in requirements.yaml file but SemVar results into different lock file, helm dep up does not replace and existing requirements.lock because requirements.yaml digest has not changed.

How to reproduce:

We have the following requirements.yaml file for a chart parent1.

dependencies:
- name: child1
  version: ">=1.0.0"
  repository: "http://127.0.0.1:8879/"
- name: child2
  version: ">=1.0.0"
  repository: "http://127.0.0.1:8879/"

When we run dep up for the first time it generates following lock file (assuming that we have child1-1.0.0 and child2-1.0.0 charts at that point)

dependencies:
- name: child1
  repository: http://127.0.0.1:8879/
  version: 1.0.0
- name: child2
  repository: http://127.0.0.1:8879/
  version: 1.0.0
digest: sha256:19b7f91a0fdfc44e14a86e7b3bbfbce2014e063bdc5949c2ad6a5edb113c3b77
generated: 2017-07-27T12:00:08.036838191+05:30

Now if we release a new version of child1 (say 1.1.0) we don’t need to update our requirements.yaml file but we expect helm dep up to update requirements.lock file accordingly.

Instead, helm dep up updates /charts folder with child1-1.1.0 but does not update requirements.lock file on disk.

When we looked into code, we figured that requirements.lock only get updated if requirements.yaml gets changed.

// Hash requirements.yaml
hash, err := resolver.HashReq(req)
if err != nil {
         return err
}

// Now we need to find out which version of a chart best satisfies the
// requirements the requirements.yaml
lock, err := m.resolve(req, repoNames, hash)
if err != nil {
	return err
}

// If the lock file hasn't changed, don't write a new one.
oldLock, err := chartutil.LoadRequirementsLock(c)
if err == nil && oldLock.Digest == lock.Digest {
	return nil
}

This results into inconsistent state of having different charts in /charts directory and requirements.lock file. Only work around, currently, is to delete requirements.lock file when we do helm dep up