gitk: Support -G option from the command line

The -G option's usage is exactly analogous to that of -S, so
supporting it is easy.
Signed-off-by: NThomas Rast <trast@inf.ethz.ch>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

gitk

@@ -227,7 +227,7 @@ proc parseviewargs {n arglist} {
 	    "--until=*" - "--before=*" - "--max-age=*" - "--min-age=*" -
 	    "--author=*" - "--committer=*" - "--grep=*" - "-[iE]" -
 	    "--remove-empty" - "--first-parent" - "--cherry-pick" -
-	    "-S*" - "--pickaxe-all" - "--pickaxe-regex" -
+	    "-S*" - "-G*" - "--pickaxe-all" - "--pickaxe-regex" -
 	    "--simplify-by-decoration" {
 		# These mean that we get a subset of the commits
 		set filtered 1