directory based gitconfig

.gitconfig

[user]
  name = a
  email = a@foo.bar
[includeIf "gitdir:~/work/"]
  path = ~/work/.gitconfig-work

.gitconfig-work

[user]
  name = b
  email = b@foo.bar

ssh-config

# default user
Host github.com-a
  Hostname github.com
  User git
  IdentityFile ~/.ssh/a.pub

# work user
Host github.com-b
  Hostname github.com
  User git
  IdentityFile ~/.ssh/b.pub

# nullroute default domain
Host github.com
  Hostname 100.127.255.255