CaseConflicts: Only check against existing files if not initial commit

[jawshooah]

Otherwise you get fatal: Not a valid object name HEAD since git ls-tree requires an existing rev.

This PR also exposes HookContext::PreCommit#initial_commit? as a public method with a corresponding delegator to make it accessible to hooks.

[jawshooah]

Looks like initial_commit? must also be exposed by the RunAll hook context for this to work. @sds, is it worth the code duplication to add that, or should we just use Overcommit::GitRepo.initial_commit? in pre-commit hooks?

[sds]

Given there are very few built-in hooks that use inital_commit?, your suggestion of having them call Overcommit::GitRepo.initial_commit? seems reasonable here.

[jawshooah]

Agreed, pushed the fix. Should be good to go.

[sds]

Merged in d898ff5. Thanks for the fix!