Use `gitoxide` to more reliably find the Git executable on Windows. (#1419)

[Byron]

Previously, Windows installations that didn't put git.exe into their PATH were unable to have their executable found.

gitoxide has the same problem to solve and can now 'find harder' in known installation locations.

This capability is now used here, instead of hard-coding git.exe or git.

Tasks

• [x] validate it works on Windows without Git in path

Fixes #4142

[krlvi]

gitbutler <3 gitoxide