strbuf: add a helper function to call the editor "on an strbuf"

strbuf: add a helper function to call the editor "on an strbuf"

This helper supports the scenario where Git has a populated `strbuf` and
wants to let the user edit it interactively.

In `git add -p`, we will use this to allow interactive hunk editing: the
diff hunks are already in memory, but we need to write them out to a
file so that an editor can be launched, then read everything back once
the user is done editing.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>