fetchGit: fetch shallow, aka depth=1

This was brought up in NixOS#1837 and seems sensible.

The issue talks about implementing a fallback, in case a shallow fetch
fails. This was not implemented so far, because I have not yet run into
a situation where it was necessary.

src/libexpr/primops/fetchGit.cc

@@ -128,7 +128,7 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
 
         // FIXME: git stderr messes up our progress indicator, so
         // we're using --quiet for now. Should process its stderr.
-        runProgram("git", true, { "-C", cacheDir, "fetch", "--quiet", "--force", "--", uri, fmt("%s:%s", *ref, *ref) });
+        runProgram("git", true, { "-C", cacheDir, "fetch", "--depth=1", "--quiet", "--force", "--", uri, fmt("%s:%s", *ref, *ref) });
 
         struct timeval times[2];
         times[0].tv_sec = now;