Feature: configurable Nim upstream repo (#76)

* add tunable repo URL when using NIM_COMMIT

* fix docs

* remove embedded single quotes from repo default

* drop old upstream (if any) before adding a new one

* rename NIM_REPO -> NIM_COMMIT_REPO

* add new remote named "extra" if upstream already exists

* remove previous "extra" remote before trying to reset it

* make extra point to custom repo, upstream to nim-lang's

README.md

@@ -169,6 +169,20 @@ You also need to specify it when using this non-default Nim compiler version:
 
 `make -j8 NIM_COMMIT="v1.2.6" nimbus_beacon_node`
 
+### NIM_COMMIT_REPO
+
+`NIM_COMMIT` will try to fetch commits from the 
+[status-im fork](https://github.com/status-im/Nim), followed by the
+[official Nim language repo](https://github.com/nim-lang/Nim). If you want to
+use a fork from somewhere else, you can set this to the repo's URL.
+
+`make -j8 NIM_COMMIT="4561f01" NIM_COMMIT_REPO="https://github.com/myorg/my-nim-fork"`
+
+As before, you will need to specify this again when using this non-default Nim
+compiler version:
+
+`make -j8 NIM_COMMIT="4561f01" NIM_COMMIT_REPO="https://github.com/myorg/my-nim-fork" nimbus_beacon_node`
+
 ### EXCLUDED_NIM_PACKAGES
 
 List of relative paths (incomplete ones also work) to Git submodules that

scripts/build_nim.sh

@@ -62,7 +62,14 @@ nim_needs_rebuilding() {
 		if ! git checkout -q ${NIM_COMMIT} 2>/dev/null; then
 			# Pay the price for a non-default NIM_COMMIT here, by fetching everything.
 			# (This includes upstream branches and tags that might be missing from our fork.)
-			git remote add upstream https://github.com/nim-lang/Nim
+			if ! git remote | grep -q "^upstream$"; then
+				git remote add upstream https://github.com/nim-lang/Nim
+			fi
+			# If the user has specified a custom repo, add it here as a remote as well.
+			if [[ -n "${NIM_COMMIT_REPO}" ]]; then
+				git remote remove extra 2>/dev/null || true
+				git remote add extra "${NIM_COMMIT_REPO}"
+			fi
 			git fetch --all --tags --quiet
 			git checkout -q ${NIM_COMMIT}
 		fi