get_base_dir returns wrong path with top level directories on windows

[theraot]

Godot version

3.2.4.beta5

System information

Windows 10

Issue description

get_base_dir seems to be returning some paths wrong.

print("C:\\puff".get_base_dir()) # outputs "C:\p"
print("C:/puff".get_base_dir()) # outputs "C:/p"

I tested multiple versions of Godot.

These return "C:" : 3.0.stable, 3.2.stable, 3.2.2.stable, 3.2.3.stable, 3.2.4.beta3, 3.2.4.beta4
These fail※: 4.0.dev※※, 3.4.beta4, 3.4.beta3, 3.3.3.stable, 3.3.2.stable, 3.2.4.beta6, 3.2.4.beta5

※: Meaning they return what I said in comments above.
※※: Built 13bd020

Thus, something went wrong from 3.2.4.beta4 to 3.2.4.beta5. I went over the commits between them, that lead me here: cddff04

Steps to reproduce

Run this code:

print("C:\\puff".get_base_dir()) # outputs "C:\p"
print("C:/puff".get_base_dir()) # outputs "C:/p"

Minimal reproduction project

No response

[theraot]

In figuring out the origin of the bug, I got very close to solving it, so I went ahead and made the pull request #52049 which I hope it can be cherry-picked to 3.x. It just changes one character in the source.