gitk: fix a 'continue' statement outside a loop to 'return'

When 5de460a2cf (gitk: Refactor per-line part of getblobdiffline and
its support) moved the body of a loop into a separate function, several
'continue' statements were changed to 'return'. But one instance was
missed. Fix it now.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>

gitk

@@ -8296,7 +8296,7 @@ proc parseblobdiffline {ids line} {
         if {![regexp {^diff (--cc|--git) } $line m type]} {
             set line [convertfrom utf-8 $line]
             $ctext insert end "$line\n" hunksep
-            continue
+            return
         }
         # start of a new file
         set diffinhdr 1