pretty: expand tabs in indented logs to make things line up properly

A commit log message sometimes tries to line things up using tabs,
assuming fixed-width font with the standard 8-place tab settings.
Viewing such a commit however does not work well in "git log", as
we indent the lines by prefixing 4 spaces in front of them.

This should all line up:

  Column 1	Column 2
  --------	--------
  A		B
  ABCD		EFGH
  SPACES        Instead of Tabs

Even with multi-byte UTF8 characters:

  Column 1	Column 2
  --------	--------
  Ä		B
  åäö		100
  A Møøse	once bit my sister..

Tab-expand the lines in "git log --expand-tabs" output before
prefixing 4 spaces.

This is based on the patch by Linus Torvalds, but at this step, we
require an explicit command line option to enable the behaviour.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.h

@@ -147,6 +147,7 @@ struct pretty_print_context {
 	int preserve_subject;
 	struct date_mode date_mode;
 	unsigned date_mode_explicit:1;
+	unsigned expand_tabs_in_log:1;
 	int need_8bit_cte;
 	char *notes_message;
 	struct reflog_walk_info *reflog_info;