fetchgit: add preFetch hook

This allows running code to change the environment before the
nix-prefetch-git script is run.  In particular, it allows setting things
like NIX_PREFETCH_GIT_CHECKOUT_HOOK, which allows additional code to be
run before deleting the .git directory.

This also means there's potentially value in performing a deep clone
then removing the .git directory, so remove the assertion that prevents
doing so.

doc/build-helpers/fetchers.chapter.md

@@ -773,9 +773,14 @@ Additionally, the following optional arguments can be given:
 
 : 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)