fetchgit: add preFetch hook (#404488)

doc/build-helpers/fetchers.chapter.md

···
 
 : Whether to fetch LFS objects.
 
+*`preFetch`* (String)
+
+: Shell code to be executed before the repository has been fetched, to allow
+  changing the environment the fetcher runs in.
+
 *`postFetch`* (String)
 
-: Shell code executed after the file has been fetched successfully.
+: Shell code executed after the repository has been fetched successfully.
   This can do things like check or transform the file.
 
 *`leaveDotGit`* (Boolean)

pkgs/build-support/fetchgit/builder.sh

···
 
 echo "exporting $url (rev $rev) into $out"
 
+runHook preFetch
+
 $SHELL $fetcher --builder --url "$url" --out "$out" --rev "$rev" --name "$name" \
   ${leaveDotGit:+--leave-dotGit} \
   ${fetchLFS:+--fetch-lfs} \

pkgs/build-support/fetchgit/default.nix

···
       nonConeMode ? false,
       name ? null,
       nativeBuildInputs ? [ ],
+      # Shell code executed before the file has been fetched.  This, in
+      # particular, can do things like set NIX_PREFETCH_GIT_CHECKOUT_HOOK to
+      # run operations between the checkout completing and deleting the .git
+      # directory.
+      preFetch ? "",
       # Shell code executed after the file has been fetched
       # successfully. This can do things like check or transform the file.
       postFetch ? "",
···
       server admins start using the new version?
     */
 
-    assert deepClone -> leaveDotGit;
     assert nonConeMode -> (sparseCheckout != [ ]);
 
     let
···
           deepClone
           branchName
           nonConeMode
+          preFetch
           postFetch
           ;
         rev = revWithTag;