nix-prefetch-git: add --name parameter

This allows tools like `npins` to avoid re-downloading pre-fetched sources at evaluation time,
by setting the same symbolic store path name as `builtins.fetchGit` does,
and would make hacks such as [0] obsolete.

Historical note: `builtins.fetchGit` was introduced in 2016 [1] and
released in 2018 [2]. Before that, there was only the Nixpkgs `fetchgit`,
and the convention for using constant store path names [3] was not established yet [4].

While arguably in retrospect store path names were a mistake, we cannot
undo it here even partially by setting them to a constant, as it would invalidate
countless fixed-output derivations and thus incur an increase in
resource consumption and potentially a large number of build failures.

[0]: f0449d090c
[1]: 38539b943a
[2]: https://github.com/NixOS/nix/releases/tag/2.0
[3]: db00265e81/source/guides/best-practices.md (L263-L264)
[4]: a8603605aa

pkgs/build-support/fetchgit/builder.sh

@@ -6,7 +6,7 @@
 
 echo "exporting $url (rev $rev) into $out"
 
-$SHELL $fetcher --builder --url "$url" --out "$out" --rev "$rev" \
+$SHELL $fetcher --builder --url "$url" --out "$out" --rev "$rev" --name "$name" \
   ${leaveDotGit:+--leave-dotGit} \
   ${fetchLFS:+--fetch-lfs} \
   ${deepClone:+--deepClone} \

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

@@ -47,6 +47,7 @@ Options:
       --url url       Any url understood by 'git clone'.
       --rev ref       Any sha1 or references (such as refs/heads/master)
       --hash h        Expected hash.
+      --name n        Symbolic store path name to use for the result (default: based on URL)
       --branch-name   Branch name to check out into
       --sparse-checkout Only fetch and checkout part of the repository.
       --non-cone-mode Use non-cone mode for sparse checkouts.
@@ -75,6 +76,7 @@ for arg; do
             --url) argfun=set_url;;
             --rev) argfun=set_rev;;
             --hash) argfun=set_hashType;;
+            --name) argfun=set_symbolicName;;
             --branch-name) argfun=set_branchName;;
             --deepClone) deepClone=true;;
             --sparse-checkout) argfun=set_sparseCheckout;;
@@ -424,6 +426,12 @@ if test -z "$branchName"; then
     branchName=fetchgit
 fi
 
+if [ -v symbolicName ]; then
+    storePathName="$symbolicName"
+else
+    storePathName="$(url_to_name "$url" "$rev")"
+fi
+
 tmpHomePath="$(mktemp -d "${TMPDIR:-/tmp}/nix-prefetch-git-tmp-home-XXXXXXXXXX")"
 exit_handlers+=(remove_tmpHomePath)
 ln -s "${NETRC:-$HOME/.netrc}" "$tmpHomePath/.netrc"
@@ -443,7 +451,7 @@ else
     # If the hash was given, a file with that hash may already be in the
     # store.
     if test -n "$expHash"; then
-        finalPath=$(nix-store --print-fixed-path --recursive "$hashType" "$expHash" "$(url_to_name "$url" "$rev")")
+        finalPath=$(nix-store --print-fixed-path --recursive "$hashType" "$expHash" "$storePathName")
         if ! nix-store --check-validity "$finalPath" 2> /dev/null; then
             finalPath=
         fi
@@ -458,7 +466,7 @@ else
         tmpPath="$(realpath "$(mktemp -d --tmpdir git-checkout-tmp-XXXXXXXX)")"
         exit_handlers+=(remove_tmpPath)
 
-        tmpFile="$tmpPath/$(url_to_name "$url" "$rev")"
+        tmpFile="$tmpPath/$storePathName"
         mkdir -p "$tmpFile"
 
         # Perform the checkout.