chore(3p/git): Unvendor git and track patches instead

This was vendored a long time ago under the expectation that keeping
it in sync with cgit would be easier this way, but it has proven not
to be a big issue.

On the other hand, a vendored copy of git is an annoying maintenance
burden. It is much easier to rebase the single (dottime) patch that we
have.

This removes the vendored copy of git and instead passes the git
source code to cgit via `pkgs.srcOnly`, which includes the applied
patch so that cgit can continue rendering dottime.

Change-Id: If31f62dea7ce688fd1b9050204e9378019775f2b

third_party/git/default.nix

@@ -1,14 +1,9 @@
-# Use the upstream git derivation (there's a lot of stuff happening in
-# there!) and just override the source:
+# git with custom patches. This is also used by cgit via
+# `pkgs.srcOnly`.
 { pkgs, ... }:
 
-(pkgs.git.overrideAttrs(_: {
-  version = "2.29.2";
-  src = ./.;
-  doInstallCheck = false;
-  preConfigure = ''
-    ${pkgs.autoconf}/bin/autoreconf -i
-  '';
-})).override {
-  sendEmailSupport = true;
-}
+pkgs.git.overrideAttrs(old: {
+  patches = (old.patches or []) ++ [
+    ./0001-feat-third_party-git-date-add-dottime-format.patch
+  ];
+})