fetchgit: split off deterministic-git functions

pkgs/build-support/fetchgit/deterministic-git

@@ -0,0 +1,45 @@
+#!/bin/sh
+
+# some git commands print to stdout, which would contaminate our JSON output
+clean_git(){
+    git "$@" >&2
+}
+
+# Remove all remote branches, remove tags not reachable from HEAD, do a full
+# repack and then garbage collect unreferenced objects.
+make_deterministic_repo(){
+    local repo="$1"
+
+    # run in sub-shell to not touch current working directory
+    (
+    cd "$repo"
+    # Remove files that contain timestamps or otherwise have non-deterministic
+    # properties.
+    rm -rf .git/logs/ .git/hooks/ .git/index .git/FETCH_HEAD .git/ORIG_HEAD \
+        .git/refs/remotes/origin/HEAD .git/config
+
+    # Remove all remote branches.
+    git branch -r | while read -r branch; do
+        clean_git branch -rD "$branch"
+    done
+
+    # Remove tags not reachable from HEAD. If we're exactly on a tag, don't
+    # delete it.
+    maybe_tag=$(git tag --points-at HEAD)
+    git tag --contains HEAD | while read -r tag; do
+        if [ "$tag" != "$maybe_tag" ]; then
+            clean_git tag -d "$tag"
+        fi
+    done
+
+    # Do a full repack. Must run single-threaded, or else we lose determinism.
+    clean_git config pack.threads 1
+    clean_git repack -A -d -f
+    rm -f .git/config
+
+    # Garbage collect unreferenced objects.
+    # Note: --keep-largest-pack prevents non-deterministic ordering of packs
+    #   listed in .git/objects/info/packs by only using a single pack
+    clean_git gc --prune=all --keep-largest-pack
+    )
+}