Merge branch 'jm/maint-strncpy-diff-no-index'

The "diff --no-index" codepath used limited-length buffers,
risking pathnames getting truncated.  Update it to use the
strbuf API.

By Jim Meyering (1) and Junio C Hamano (1)
* jm/maint-strncpy-diff-no-index:
  diff --no-index: use strbuf for temporary pathnames
  diff: avoid stack-buffer-read-overrun for very long name