Can't use SHA as a whippet ref

[RobjS]

It would be useful if we could use a specific commit SHA to pin a dependency to specific commit in whippet.json, e.g.:

{
  "name": "dxw-digest",
  "ref": "somecommitSHAhere"
}

However, this produces an error when running whippet deps update:

ERROR: git command failed: ref not found

This is because the git module uses git ls-remote:

whippet/src/Git/Git.php

Line 343 in b3f6d4f

exec(sprintf('git ls-remote %s %s', escapeshellarg($repo), escapeshellarg($ref)), $output, $return);

This won't produce any output for a SHA reference, so it fails. We should find a way of handling SHA references.

[jcoglan]

The rev-parse command interprets any kind of revision and returns a commit ID, if the object exists, for example:

$ git rev-parse origin/master
9c971bf308735afbf920743c347d99533762018b

$ git rev-parse 9c971bf308735afbf920743c347d99533762018b
9c971bf308735afbf920743c347d99533762018b

This includes the ability to expand abbreviated SHAs, e.g.:

$ git rev-parse 9c971bf3
9c971bf308735afbf920743c347d99533762018b

[RobjS]

Unfortunately rev-parse only works on a repo that has already been cloned locally. I guess we're using ls-remote here because the intention is to check whether the ref exists in the remote repository before it's cloned.

So far, I'm not sure git actually has a way of letting you check for the existence of a commit ID on a remote without first cloning it. So this may not be possible.