gitk: Teach gitk to respect log.showroot

gitk: Teach gitk to respect log.showroot

In early days, all projects managed by git (except for git itself) had the
product of a fairly mature development history in their first commit, and
it was deemed unnecessary clutter to show additions of these thousands of
paths as a patch.

"git log" learned to show the patch for the initial commit without requiring
--root command line option at 0f03ca9 (config option log.showroot to show
the diff of root commits, 2006-11-23).

Teach gitk to respect log.showroot.

[paulus@samba.org: Cleaned up the Tcl a bit, use --bool on the
 git config call]

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Signed-off-by: Paul Mackerras <paulus@samba.org>