Merge branch 'rs/strbuf-insertstr'

Code clean-up.

* rs/strbuf-insertstr:
  mailinfo: don't insert header prefix for handle_content_type()
  strbuf: add and use strbuf_insertstr()

diff --cc builtin/checkout.c
Simple merge

diff --cc builtin/sparse-checkout.c
Simple merge

diff --cc commit.c
Simple merge

diff --cc notes.c
Simple merge

diff --cc pretty.c
Simple merge