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>

builtin/log.c

@@ -329,8 +329,7 @@ static void show_tagger(char *buf, int len, struct rev_info *rev)
 	struct strbuf out = STRBUF_INIT;
 
 	pp_user_info("Tagger", rev->commit_format, &out, buf, rev->date_mode,
-		git_log_output_encoding ?
-		git_log_output_encoding: git_commit_encoding);
+		get_log_output_encoding());
 	printf("%s", out.buf);
 	strbuf_release(&out);
 }