commit: helper methods to reduce redundant blocks of code

commit: helper methods to reduce redundant blocks of code

* builtin/commit.c: Replace block of code with a one-liner call to
  logmsg_reencode().

* commit.c: new function for looking up a comit by name

* pretty.c: helper methods for getting output encodings

  Add helpers get_log_output_encoding() and
  get_commit_output_encoding() that eliminate some messy and duplicate
  if-blocks.

Signed-off-by: Pat Notz <patnotz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>