fetchgit: Support sparse checkout

This allow git checkout small parts of a large repo, and avoid fetching
unnecessary blobs from server.

pkgs/build-support/fetchgit/builder.sh

@@ -11,6 +11,7 @@ $SHELL $fetcher --builder --url "$url" --out "$out" --rev "$rev" \
   ${fetchLFS:+--fetch-lfs} \
   ${deepClone:+--deepClone} \
   ${fetchSubmodules:+--fetch-submodules} \
+  ${sparseCheckout:+--sparse-checkout "$sparseCheckout"} \
   ${branchName:+--branch-name "$branchName"}
 
 runHook postFetch

pkgs/build-support/fetchgit/default.nix

@@ -15,6 +15,7 @@ in
 { url, rev ? "HEAD", md5 ? "", sha256 ? "", hash ? "", leaveDotGit ? deepClone
 , fetchSubmodules ? true, deepClone ? false
 , branchName ? null
+, sparseCheckout ? ""
 , name ? urlToName url rev
 , # Shell code executed after the file has been fetched
   # successfully. This can do things like check or transform the file.
@@ -74,7 +75,7 @@ stdenvNoCC.mkDerivation {
   else
     lib.fakeSha256;
 
-  inherit url rev leaveDotGit fetchLFS fetchSubmodules deepClone branchName postFetch;
+  inherit url rev leaveDotGit fetchLFS fetchSubmodules deepClone branchName sparseCheckout postFetch;
 
   postHook = if netrcPhase == null then null else ''
     ${netrcPhase}

pkgs/build-support/fetchgit/nix-prefetch-git

@@ -48,6 +48,7 @@ Options:
       --rev ref       Any sha1 or references (such as refs/heads/master)
       --hash h        Expected hash.
       --branch-name   Branch name to check out into
+      --sparse-checkout Only fetch and checkout part of the repository.
       --deepClone     Clone the entire repository.
       --no-deepClone  Make a shallow clone of just the required ref.
       --leave-dotGit  Keep the .git directories.
@@ -75,6 +76,7 @@ for arg; do
             --hash) argfun=set_hashType;;
             --branch-name) argfun=set_branchName;;
             --deepClone) deepClone=true;;
+            --sparse-checkout) argfun=set_sparseCheckout;;
             --quiet) QUIET=true;;
             --no-deepClone) deepClone=;;
             --leave-dotGit) leaveDotGit=true;;
@@ -96,7 +98,7 @@ for arg; do
         case $argfun in
             set_*)
                 var=${argfun#set_}
-                eval $var=$arg
+                eval "$var=$(printf %q "$arg")"
                 ;;
         esac
         argfun=""
@@ -112,6 +114,10 @@ init_remote(){
     local url=$1
     clean_git init --initial-branch=master
     clean_git remote add origin "$url"
+    if [ -n "$sparseCheckout" ]; then
+        git config remote.origin.partialclonefilter "blob:none"
+        echo "$sparseCheckout" | git sparse-checkout set --stdin
+    fi
     ( [ -n "$http_proxy" ] && clean_git config http.proxy "$http_proxy" ) || true
 }