fix(forge): init a git repo at root during init unless explicitely stated

[0xvv]

Motivation

Fixes: #3962

Solution

Force initialization of a git repo at the root of the foundry folder unless there already is one or the flag --use-parent-git is present and there is indeed a parent git repo.

The --use-parent-git flag replicates the current behavior in case some people prefer it.

Tested the reproductions steps of the issue after to be sure.
The fix in forge init leads to the expected behavior for forge install

[grandizzy]

I am not sure I like the way git submodules is desired to be used in #3962 (could become beefy and hard to maintain) but rather have different projects / deps with their own foundry.toml file and submodules. @zerosnacks @yash-atreya wdyt?