diff --stat: add config option to limit graph width

Config option diff.statGraphWidth=<width> is equivalent to
--stat-graph-width=<width>, except that the config option is ignored
by format-patch.

For the graph-width limiting to be usable, it should happen
'automatically' once configured, hence the config option.
Nevertheless, graph width limiting only makes sense when used on a
wide terminal, so it should not influence the output of format-patch,
which adheres to the 80-column standard.

Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2118,6 +2118,7 @@ _git_config ()
 		core.whitespace
 		core.worktree
 		diff.autorefreshindex
+		diff.statGraphWidth
 		diff.external
 		diff.ignoreSubmodules
 		diff.mnemonicprefix