Merge legacy fetchGit-builtin with the generic fetchTree-function

The original idea was to implement a git-fetcher in Nix's core that
supports content hashes[1]. In #3549[2] it has been suggested to
actually use `fetchTree` for this since it's a fairly generic wrapper
over the new fetcher-API[3] and already supports content-hashes.

This patch implements a new git-fetcher based on `fetchTree` by
incorporating the following changes:

* Removed the original `fetchGit`-implementation and replaced it with an
  alias on the `fetchTree` implementation.

* Ensured that the `git`-fetcher from `libfetchers` always computes a
  content-hash and returns an "empty" revision on dirty trees (the
  latter one is needed to retain backwards-compatibility).

* The hash-mismatch error in the fetcher-API exits with code 102 as it
  usually happens whenever a hash-mismatch is detected by Nix.

* Removed the `flakes`-feature-flag: I didn't see a reason why this API
  is so tightly coupled to the flakes-API and at least `fetchGit` should
  remain usable without any feature-flags.

* It's only possible to specify a `narHash` for a `git`-tree if either a
  `ref` or a `rev` is given[4].

* It's now possible to specify an URL without a protocol. If it's missing,
  `file://` is automatically added as it was the case in the original
  `fetchGit`-implementation.

[1] https://github.com/NixOS/nix/pull/3216
[2] https://github.com/NixOS/nix/pull/3549#issuecomment-625194383
[3] https://github.com/NixOS/nix/pull/3459
[4] https://github.com/NixOS/nix/pull/3216#issuecomment-553956703

tests/fetchGit.sh

@@ -32,6 +32,8 @@ rev2=$(git -C $repo rev-parse HEAD)
 # Fetch a worktree
 unset _NIX_FORCE_HTTP
 path0=$(nix eval --impure --raw --expr "(builtins.fetchGit file://$TEST_ROOT/worktree).outPath")
+path0_=$(nix eval --impure --raw --expr "(builtins.fetchTree { type = \"git\"; url = file://$TEST_ROOT/worktree; }).outPath")
+[[ $path0 = $path0_ ]]
 export _NIX_FORCE_HTTP=1
 [[ $(tail -n 1 $path0/hello) = "hello" ]]
 
@@ -87,8 +89,6 @@ path2=$(nix eval --impure --raw --expr "(builtins.fetchGit $repo).outPath")
 [ ! -e $path2/.git ]
 [[ $(cat $path2/dir1/foo) = foo ]]
 
-[[ $(nix eval --impure --raw --expr "(builtins.fetchGit $repo).rev") = 0000000000000000000000000000000000000000 ]]
-
 # ... unless we're using an explicit ref or rev.
 path3=$(nix eval --impure --raw --expr "(builtins.fetchGit { url = $repo; ref = \"master\"; }).outPath")
 [[ $path = $path3 ]]
@@ -102,6 +102,12 @@ git -C $repo commit -m 'Bla3' -a
 path4=$(nix eval --impure --refresh --raw --expr "(builtins.fetchGit file://$repo).outPath")
 [[ $path2 = $path4 ]]
 
+nix eval --impure --raw --expr "(builtins.fetchGit { url = $repo; rev = \"$rev2\"; narHash = \"sha256-B5yIPHhEm0eysJKEsO7nqxprh9vcblFxpJG11gXJus1=\"; }).outPath" || status=$?
+[[ "$status" = "102" ]]
+
+path5=$(nix eval --impure --raw --expr "(builtins.fetchGit { url = $repo; rev = \"$rev2\"; narHash = \"sha256-Hr8g6AqANb3xqX28eu1XnjK/3ab8Gv6TJSnkb1LezG9=\"; }).outPath")
+[[ $path = $path5 ]]
+
 # tarball-ttl should be ignored if we specify a rev
 echo delft > $repo/hello
 git -C $repo add hello
@@ -123,7 +129,7 @@ path2=$(nix eval --impure --raw --expr "(builtins.fetchGit file://$repo).outPath
 # Using local path with branch other than 'master' should work when clean or dirty
 path3=$(nix eval --impure --raw --expr "(builtins.fetchGit $repo).outPath")
 # (check dirty-tree handling was used)
-[[ $(nix eval --impure --raw --expr "(builtins.fetchGit $repo).rev") = 0000000000000000000000000000000000000000 ]]
+[[ $(nix eval --impure --expr "(builtins.fetchGit $repo).rev or null") = null ]]
 
 # Committing shouldn't change store path, or switch to using 'master'
 git -C $repo commit -m 'Bla5' -a