Add fetchMercurial primop

E.g.

  $ nix eval '(fetchMercurial https://www.mercurial-scm.org/repo/hello)'
  { branch = "default"; outPath = "/nix/store/alvb9y1kfz42bjishqmyy3pphnrh1pfa-source"; rev = "82e55d328c8ca4ee16520036c0aaace03a5beb65"; revCount = 1; shortRev = "82e55d328c8c"; }

  $ nix eval '(fetchMercurial { url = https://www.mercurial-scm.org/repo/hello; rev = "0a04b987be5ae354b710cefeba0e2d9de7ad41a9"; })'
  { branch = "default"; outPath = "/nix/store/alvb9y1kfz42bjishqmyy3pphnrh1pfa-source"; rev = "0a04b987be5ae354b710cefeba0e2d9de7ad41a9"; revCount = 0; shortRev = "0a04b987be5a"; }

  $ nix eval '(fetchMercurial /tmp/unclean-hg-tree)'
  { branch = "default"; outPath = "/nix/store/cm750cdw1x8wfpm3jq7mz09r30l9r024-source"; rev = "0000000000000000000000000000000000000000"; revCount = 0; shortRev = "000000000000"; }

1 files changed, 1 insertions, 2 deletions

diff --git a/src/libexpr/primops/fetchgit.cc b/src/libexpr/primops/fetchgit.cc
index dc526485fa31..879e3656a25a 100644
--- a/src/libexpr/primops/fetchgit.cc
+++ b/src/libexpr/primops/fetchgit.cc
@@ -106,10 +106,9 @@ GitInfo exportGit(ref<Store> store, const std::string & uri,
 
     std::string storeLinkName = hashString(htSHA512, name + std::string("\0"s) + gitInfo.rev).to_string(Base32, false);
     Path storeLink = cacheDir + "/" + storeLinkName + ".link";
-    PathLocks storeLinkLock({storeLink}, fmt("waiting for lock on '%1%'...", storeLink));
+    PathLocks storeLinkLock({storeLink}, fmt("waiting for lock on '%1%'...", storeLink)); // FIXME: broken
 
     try {
-        // FIXME: doesn't handle empty lines
         auto json = nlohmann::json::parse(readFile(storeLink));
 
         assert(json["name"] == name && json["rev"] == gitInfo.rev);