gitk: Fix handling of tree file list with special chars in names

gitk: Fix handling of tree file list with special chars in names

Alex Riesen pointed out that displaying a commit in 'tree' mode fails
if some files have names with special characters such as '{' or '}' in
them, due to the fact that we treat the line returned from git ls-tree
as a Tcl list at one point.

This fixes it by doing what I originally intended but didn't quite
get right.  We split the line from git ls-tree at the first tab and
treat the part before the tab as a list (which is OK since it doesn't
have special characters in it) and the part after the tab as the
filename.

Signed-off-by: Paul Mackerras <paulus@samba.org>