Merge branch 'jk/mbox-from-line'

Some MUAs mangled a line in a message that begins with "From " to
">From " when writing to a mailbox file and feeding such an input
to "git am" used to lose such a line.

* jk/mbox-from-line:
  mailinfo: work around -Wstring-plus-int warning
  mailinfo: make ">From" in-body header check more robust