API, Core: Fix naming in fastForwardBranch/replaceBranch APIs

[amogh-jahagirdar]

This change fixes some very confusing naming from my original implementation of the fastForward/replaceBranch APIs.

Prior to this change, the APIs had the following naming:

fastForward(String name, String source)
replaceBranch(String name, String source)

"Name" is the branch that would be fast forwarded and source is the "destination" or target we would fast forward to. This doesn't really align with how most people would think when it comes to fast forward or replace operations (like Git).

This change renames it to:

fastForward(String from, String to)
replaceBranch(String from, String to)

Note: This is just a rename of parameters and error messages. There is no change in behavior in the API.

[amogh-jahagirdar]

cc @rakesh-das08@singhpk234 This stems from our discussion on #8854 (comment)

[nastra]

I think we're a little inconsistent here in terms of null checking (throwing NPE vs IAE) but I guess that's out-of-scope for this PR, so just wanted to mention that

[amogh-jahagirdar]

Yeah you're right, and this unfortunately applies to more than just replace/fastForward. (e.g. create, rename, as well throw an NPE instead of IAE). I'll take this in a follow on.

[amogh-jahagirdar]

Merging, thanks for the reviews @nastra@rakesh-das08