Merge branch 'jk/credentials'

Merge branch 'jk/credentials'

* jk/credentials:
  t: add test harness for external credential helpers
  credentials: add "store" helper
  strbuf: add strbuf_add*_urlencode
  Makefile: unix sockets may not available on some platforms
  credentials: add "cache" helper
  docs: end-user documentation for the credential subsystem
  credential: make relevance of http path configurable
  credential: add credential.*.username
  credential: apply helper config
  http: use credential API to get passwords
  credential: add function for parsing url components
  introduce credentials API
  t5550: fix typo
  test-lib: add test_config_global variant

Conflicts:
	strbuf.c