git-x: regroup CLI under tree/commit; add tree add

Previously the eight git-x subcommands sat flat at top level (split,
check, fix, verify, filter-paths, split-commit, drop-commit, reword),
with 'split' ambiguous between the subtree split and the per-directory
commit split.

Regrouped into two namespaces that mirror the object they act on:

git-x tree
split (was: git-x split)
add (new — inject a standalone history under a prefix)
drop (was: git-x filter-paths)
check (was: git-x check)
fix (was: git-x fix)
verify (was: git-x verify)

git-x commit
split (was: git-x split-commit)
drop (was: git-x drop-commit)
reword (was: git-x reword)

Each subcommand lives in cmd_<group>_<verb>.{ml,mli}; cmd_tree.ml and
cmd_commit.ml are the Cmd.group wrappers. git_x.ml registers just the
two groups.

'tree add' is a thin wrapper over Git.Subtree.add, which already
existed in the library but had no CLI exposure. It accepts a ref (e.g.
FETCH_HEAD after 'git fetch URL REF') and a --prefix, then builds a
subtree-merge commit with the current user's git config identity.

Log source names are updated to match (git-x.tree.split,
git-x.tree.fix). The cram test under test/cram/tree_split.t is
updated to use the new 'git-x tree split' invocation throughout.