Merge branch 'rp/maintenance-qol'

Merge branch 'rp/maintenance-qol'

'git maintenance register' is taught to write configuration to an
arbitrary path, and 'git for-each-repo' is taught to expand tilde
characters in paths.

* rp/maintenance-qol:
  builtin/gc.c: fix use-after-free in maintenance_unregister()
  maintenance --unregister: fix uninit'd data use & -Wdeclaration-after-statement
  maintenance: add option to register in a specific config
  for-each-repo: interpolate repo path arguments