gitk: Fix bug causing Tcl error when updating graph

gitk: Fix bug causing Tcl error when updating graph

If "Show nearby tags" is turned off, selecting "Update" from the File
menu will cause a Tcl error.  This fixes it.  The problem was that
we were calling regetallcommits unconditionally, but it assumed that
getallcommits had been called previously.  This also restructures
{re,}getallcommits to be a bit simpler.

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