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

"diff --no-index" codepath had a few places that used fixed-size
buffer and truncated paths that are too long.

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