Merge branch 'cw/strbuf-cleanup'

Merge branch 'cw/strbuf-cleanup'

Move functions that are not about pure string manipulation out of
strbuf.[ch]

* cw/strbuf-cleanup:
  strbuf: remove global variable
  path: move related function to path
  object-name: move related functions to object-name
  credential-store: move related functions to credential-store file
  abspath: move related functions to abspath
  strbuf: clarify dependency
  strbuf: clarify API boundary