GitInputScheme: Use libgit2

This replaces most calls to the "git" binary with libgit2.

src/libfetchers/git-utils.hh

@@ -0,0 +1,56 @@
+#pragma once
+
+#include "input-accessor.hh"
+
+namespace nix {
+
+struct GitRepo
+{
+    virtual ~GitRepo()
+    { }
+
+    static ref<GitRepo> openRepo(const CanonPath & path, bool create = false, bool bare = false);
+
+    virtual uint64_t getRevCount(const Hash & rev) = 0;
+
+    virtual uint64_t getLastModified(const Hash & rev) = 0;
+
+    virtual bool isShallow() = 0;
+
+    /* Return the commit hash to which a ref points. */
+    virtual Hash resolveRef(std::string ref) = 0;
+
+    struct WorkdirInfo
+    {
+        bool isDirty = false;
+
+        /* The checked out commit, or nullopt if there are no commits
+           in the repo yet. */
+        std::optional<Hash> headRev;
+
+        /* All files in the working directory that are unchanged,
+           modified or added, but excluding deleted files. */
+        std::set<CanonPath> files;
+    };
+
+    virtual WorkdirInfo getWorkdirInfo() = 0;
+
+    /* Get the ref that HEAD points to. */
+    virtual std::optional<std::string> getWorkdirRef() = 0;
+
+    struct TarballInfo
+    {
+        Hash treeHash;
+        time_t lastModified;
+    };
+
+    virtual bool hasObject(const Hash & oid) = 0;
+
+    virtual ref<InputAccessor> getAccessor(const Hash & rev) = 0;
+
+    virtual void fetch(
+        const std::string & url,
+        const std::string & refspec) = 0;
+};
+
+}