Link ~/.gitconfig, not ~/.config/git/config

Git reads both, and ~/.gitconfig wins. Linking only the XDG path left the
stale ~/.gitconfig in charge, so system76 kept signing commits as a
previous employer even after the cutover appeared to succeed — the symlink
was correct and had no effect.

meta/base.conf.yaml

@@ -23,5 +23,5 @@
     ~/.config/htop:         common/htop
     ~/.config/lf:           common/lf
     ~/.config/starship.toml: common/starship.toml
-    ~/.config/git/config:   common/git/config
+    ~/.gitconfig:           common/git/config
     ~/.ssh/config:          common/ssh/config