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/mailinfo.c

@@ -1032,7 +1032,7 @@ int cmd_mailinfo(int argc, const char **argv, const char *prefix)
 	 */
 	git_config(git_mailinfo_config, NULL);
 
-	def_charset = (git_commit_encoding ? git_commit_encoding : "UTF-8");
+	def_charset = get_commit_output_encoding();
 	metainfo_charset = def_charset;
 
 	while (1 < argc && argv[1][0] == '-') {