fetchgit: add a gitConfigFile argument

Passing a `gitConfigFile` argument as a string allows setting a custom
git config (as `$GIT_CONFIG_GLOBAL`). The most obvious use is to
override URLs with custom mirrors, but others are possible too.

pkgs/build-support/fetchgit/builder.sh

@@ -8,6 +8,11 @@ echo "exporting $url (rev $rev) into $out"
 
 runHook preFetch
 
+if [ -n "$gitConfigFile" ]; then
+  echo "using GIT_CONFIG_GLOBAL=$gitConfigFile"
+  export GIT_CONFIG_GLOBAL="$gitConfigFile"
+fi
+
 $SHELL $fetcher --builder --url "$url" --out "$out" --rev "$rev" --name "$name" \
   ${leaveDotGit:+--leave-dotGit} \
   ${fetchLFS:+--fetch-lfs} \

pkgs/build-support/fetchgit/default.nix

@@ -1,6 +1,7 @@
 {
   lib,
   stdenvNoCC,
+  writeText,
   git,
   git-lfs,
   cacert,
@@ -64,6 +65,8 @@ lib.makeOverridable (
       fetchTags ? false,
       # make this subdirectory the root of the result
       rootDir ? "",
+      # GIT_CONFIG_GLOBAL (as a file)
+      gitConfigFile ? null,
     }:
 
     /*
@@ -148,6 +151,7 @@ lib.makeOverridable (
           postFetch
           fetchTags
           rootDir
+          gitConfigFile
           ;
         rev = revWithTag;
 

pkgs/build-support/fetchgit/tests.nix

@@ -7,6 +7,7 @@
   cacert,
   nix,
   closureInfo,
+  lib,
   ...
 }:
 {
@@ -194,4 +195,14 @@
         rev = "9d9dbe6ed05854e03811c361a3380e09183f4f4a";
         hash = "sha256-7DszvbCNTjpzGRmpIVAWXk20P0/XTrWZ79KSOGLrUWY=";
       };
+
+  withGitConfig = testers.invalidateFetcherByDrvHash fetchgit {
+    name = "fetchgit-with-config";
+    url = "https://doesntexist.forsure/NixOS/nix";
+    rev = "9d9dbe6ed05854e03811c361a3380e09183f4f4a";
+    sha256 = "sha256-7DszvbCNTjpzGRmpIVAWXk20P0/XTrWZ79KSOGLrUWY=";
+    gitConfigFile = builtins.toFile "gitconfig" (lib.generators.toGitINI {
+      url."https://github.com".insteadOf = "https://doesntexist.forsure";
+    });
+  };
 }