Make Git source bundle entries self-contained

Git source checkouts used internally by Lito share their object store with
the Git cache. Exporting such a checkout with --no-hardlinks alone can
preserve references to the original object store through Git alternates,
making `lito fetch --output` bundles unusable after the Lito cache is
removed.

Use --dissociate when exporting Git checkouts so all required objects are
copied into the exported repository and the source bundle remains portable.
--- ../tools/src/git.cppm	2026-09-26 22:41:11.544970987 +0300
+++ ../tools/src/git.cppm	2026-09-26 22:41:24.639646927 +0300
@@ -186,6 +186,7 @@
         arguments.push("clone"_Str);
         arguments.push("--no-checkout"_Str);
         arguments.push("--no-hardlinks"_Str);
+        arguments.push("--dissociate"_Str);
         rstd_try(command::push_path(arguments, source));
         rstd_try(command::push_path(arguments, destination));
         rstd_try(git_status(rstd::move(arguments), "Git source bundle export"_str, *environment_));