pager: add a helper function to clear the last line in the terminal

pager: add a helper function to clear the last line in the terminal

There are a couple of places where we want to clear the last line on
the terminal, e.g. when a progress bar line is overwritten by a
shorter line, then the end of that progress line would remain visible,
unless we cover it up.

In 'progress.c' we did this by always appending a fixed number of
space characters to the next line (even if it was not shorter than the
previous), but as it turned out that fixed number was not quite large
enough, see the fix in 9f1fd84e15 (progress: clear previous progress
update dynamically, 2019-04-12).  From then on we've been keeping
track of the length of the last displayed progress line and appending
the appropriate number of space characters to the next line, if
necessary, but, alas, this approach turned out to be error prone, see
the fix in 1aed1a5f25 (progress: avoid empty line when breaking the
progress line, 2019-05-19).  The next patch in this series is about to
fix a case where we don't clear the last line, and on occasion do end
up with such garbage at the end of the line.  It would be great if we
could do that without the need to deal with that without meticulously
computing the necessary number of space characters.

So add a helper function to clear the last line on the terminal using
an ANSI escape sequence, which has the advantage to clear the whole
line no matter how wide it is, even after the terminal width changed.
Such an escape sequence is not available on dumb terminals, though, so
in that case fall back to simply print a whole terminal width (as
reported by term_columns()) worth of space characters.

In 'editor.c' launch_specified_editor() already used this ANSI escape
sequence, so replace it with a call to this function.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>