fix: lerna diff for repositories without commits

[joscha]

lerna diff on a repository without any commits fails due to:

$ lerna diff
Lerna v2.0.0-beta.7
Independent Versioning Mode
fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
Errored while running DiffCommand.initialize
Error: Command failed: git rev-list --max-parents=0 HEAD
fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

    at checkExecSyncError (child_process.js:470:13)
    at Object.execSync (child_process.js:510:13)
    at Function.execSync (/usr/local/lib/node_modules/lerna/lib/ChildProcessUtilities.js:37:38)
    at Function.getFirstCommit (/usr/local/lib/node_modules/lerna/lib/GitUtilities.js:92:46)
    at DiffCommand.initialize (/usr/local/lib/node_modules/lerna/lib/commands/DiffCommand.js:63:130)
    at DiffCommand._attempt (/usr/local/lib/node_modules/lerna/lib/Command.js:148:21)
    at DiffCommand.runCommand (/usr/local/lib/node_modules/lerna/lib/Command.js:132:12)
    at DiffCommand.run (/usr/local/lib/node_modules/lerna/lib/Command.js:62:12)
    at Object.<anonymous> (/usr/local/lib/node_modules/lerna/bin/lerna.j%

currently.

[jamiebuilds]

Can you make this return a boolean instead:

!!ChildProcessUtilities.execSync("git log");

[jamiebuilds]

Thanks for tackling this @joscha

[joscha]

@thejameskyle I made the changes, let me know if that works for you. I can also squash the commits in case you'd like that.

[hzoo]

We can squash in the merge now so it's ok

[jamiebuilds]

Thanks!

[joscha]

Thank you for merging - one thing I thought with the errors we have now in quite a few places - would you be happy to integrate a PR that unifies and cleans up the errors using https://www.npmjs.com/package/define-error ?

[jamiebuilds]

Yeah, errors and error handling are definitely places I want to be focusing on. I'm not sure what that package would do to help though.,

[joscha]

I think exporting the errors and making them importable at the right places would be the first step. The package itself only makes sure that there is a proper stack trace attached.

[lock]

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