Merge branch 'jc/format-patch-reroll'

Merge branch 'jc/format-patch-reroll'

Teach "format-patch" to prefix v4- to its output files for the
fourth iteration of a patch series, to make it easier for the
submitter to keep separate copies for iterations.

* jc/format-patch-reroll:
  format-patch: give --reroll-count a short synonym -v
  format-patch: document and test --reroll-count
  format-patch: add --reroll-count=$N option
  get_patch_filename(): split into two functions
  get_patch_filename(): drop "just-numbers" hack
  get_patch_filename(): simplify function signature
  builtin/log.c: stop using global patch_suffix
  builtin/log.c: drop redundant "numbered_files" parameter from make_cover_letter()
  builtin/log.c: drop unused "numbered" parameter from make_cover_letter()