feat: add fixed point comparison for history

[cenk1cenk2]

This pull request adds a fixed point comparison for history mode.

By passing another flag with the existing flag parsing mechanism, you can designate a fixed point to compare a given commit with --base WORKING or --base HEAD.

This adds a different behavior where every commit you go back in history will only compare it against it to the designated state.

An example is as in the following case.

The current form without is as follows.

commit 5
commit 4 (commit 4 vs commit 5)
commit 3 (commit 3 vs commit 4)

with --base HEAD which we can imagine as commit 5;

commit 5 
commit 4 (commit 4 vs commit 5)
commit 3 (commit 3 vs commit 5)

This is comparable to the same-named diffview.nvim flag.