build: derive patches upstream-head ref from script path (#50727)
* build: derive patches upstream-head ref from script path
gclient-new-workdir.py symlinks each repo's .git/refs back to the source
checkout, so the fixed refs/patches/upstream-head was shared across all
worktrees. Parallel `e sync` runs in different worktrees clobbered each
other's upstream-head, breaking `e patches` and check-patch-diff.
Suffix the ref with an md5 of the script directory so each worktree writes
a distinct ref into the shared refs dir. Fall back to the legacy ref name
in guess_base_commit so existing checkouts keep working until next sync.
* fixup: also write legacy upstream-head ref and note it in docs
