blame: add config options for the output of ignored or unblamable lines

blame: add config options for the output of ignored or unblamable lines

When ignoring commits, the commit that is blamed might not be
responsible for the change, due to the inaccuracy of our heuristic.
Users might want to know when a particular line has a potentially
inaccurate blame.

Furthermore, guess_line_blames() may fail to find any parent commit for
a given line touched by an ignored commit.  Those 'unblamable' lines
remain blamed on an ignored commit.  Users might want to know if a line
is unblamable so that they do not spend time investigating a commit they
know is uninteresting.

This patch adds two config options to mark these two types of lines in
the output of blame.

The first option can identify ignored lines by specifying
blame.markIgnoredLines.  When this option is set, each blame line that
was blamed on a commit other than the ignored commit is marked with a
'?'.

For example:
	278b6158d6fdb (Barret Rhoden  2016-04-11 13:57:54 -0400 26)
appears as:
	?278b6158d6fd (Barret Rhoden  2016-04-11 13:57:54 -0400 26)

where the '?' is placed before the commit, and the hash has one fewer
characters.

Sometimes we are unable to even guess at what ancestor commit touched a
line.  These lines are 'unblamable.'  The second option,
blame.markUnblamableLines, will mark the line with '*'.

For example, say we ignore e5e8d36d04cbe, yet we are unable to blame
this line on another commit:
	e5e8d36d04cbe (Barret Rhoden  2016-04-11 13:57:54 -0400 26)
appears as:
	*e5e8d36d04cb (Barret Rhoden  2016-04-11 13:57:54 -0400 26)

When these config options are used together, every line touched by an
ignored commit will be marked with either a '?' or a '*'.

Signed-off-by: Barret Rhoden <brho@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>