Use libgit2 with ssh-exec support

See libgit2/libgit2#6617. This ensures that we
get support for ~/.ssh/config, known_hosts etc.

flake.nix

@@ -7,8 +7,9 @@
   inputs.nixpkgs-regression.url = "github:NixOS/nixpkgs/215d4d0fd80ca5163643b03a33fde804a29cc1e2";
   inputs.lowdown-src = { url = "github:kristapsdz/lowdown"; flake = false; };
   inputs.flake-compat = { url = "github:edolstra/flake-compat"; flake = false; };
+  inputs.libgit2 = { url = "github:libgit2/libgit2"; flake = false; };
 
-  outputs = { self, nixpkgs, nixpkgs-regression, lowdown-src, flake-compat }:
+  outputs = { self, nixpkgs, nixpkgs-regression, lowdown-src, flake-compat, libgit2 }:
 
     let
       inherit (nixpkgs) lib;
@@ -194,7 +195,11 @@
             bzip2 xz brotli editline
             openssl sqlite
             libarchive
-            libgit2
+            (pkgs.libgit2.overrideAttrs (attrs: {
+              src = libgit2;
+              version = libgit2.lastModifiedDate;
+              cmakeFlags = (attrs.cmakeFlags or []) ++ ["-DUSE_SSH=exec"];
+            }))
             boost
             lowdown-nix
             libsodium

src/libfetchers/git-utils.cc

@@ -336,9 +336,6 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
         const std::string & url,
         const std::string & refspec) override
     {
-        /* FIXME: use libgit2. Unfortunately, it doesn't support
-           ssh_config at the moment. */
-        #if 0
         Remote remote;
 
         if (git_remote_create_anonymous(Setter(remote), *this, url.c_str()))
@@ -352,20 +349,6 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
 
         if (git_remote_fetch(remote.get(), &refspecs2, nullptr, nullptr))
             throw Error("fetching '%s' from '%s': %s", refspec, url, git_error_last()->message);
-        #endif
-
-        // FIXME: git stderr messes up our progress indicator, so
-        // we're using --quiet for now. Should process its stderr.
-        runProgram("git", true,
-            { "-C", path.abs(),
-              "--bare",
-              "fetch",
-              "--quiet",
-              "--force",
-              "--",
-              url,
-              refspec
-            }, {}, true);
     }
 
     void verifyCommit(