Merge branch 'jt/mailinfo-fold-in-body-headers'

When "git format-patch --stdout" output is placed as an in-body
header and it uses the RFC2822 header folding, "git am" failed to
put the header line back into a single logical line.  The
underlying "git mailinfo" was taught to handle this properly.

* jt/mailinfo-fold-in-body-headers:
  mailinfo: handle in-body header continuations
  mailinfo: make is_scissors_line take plain char *
  mailinfo: separate in-body header processing