gitk: Fix error when changing colors after closing "List references" window

gitk: Fix error when changing colors after closing "List references" window

This fixes an error that manifests itself if the user opens the
"List references" window and the closes it, and subsequently opens
the Preferences window and changes one of the colors.  When the
user clicks OK, and error popup appears with the message:

Error: invalid command name ".showrefs.list"

This is because .showrefs.list was added to the list of windows to
be notified on foreground/background color changes, but the window
no longer exists.  We fix the bug by checking whether the window
exists before trying to change its colors.  As an optimization, we
also avoid adding the .showrefs.list window to the list a second
time.

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