Add OS-specific gitconfig

[fogrew]

Read an article
Something like

[includeIf "gitdir:/Users"]
    path = ~/.gitconfig-macos
[includeIf "gitdir:C:"]
    path = ~/.gitconfig-windows
[includeIf "gitdir:/home"]
    path = ~/.gitconfig-linux

for macos

[credential]
    helper = osxkeychain

for windows

[credential]
    helper = manager