gitk: Avoid crash if closed while reading references

gitk: Avoid crash if closed while reading references

As recorded in msysGit issue 125, if the user closes gitk while it
reports itself as still reading references then Tk will crash in the
geometry management code.  This has been fixed for Tk 8.5.7 and above.
This patch avoids the problem by flushing outstanding geometry events
before calling the readrefs procedure.

See also http://code.google.com/p/msysgit/issues/detail?id=125

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>