gitk: Add a cache for the topology info

This adds code to write out the topology information used to determine
precedes/follows and branch information into a cache file (~3.5MB for
the kernel tree).  At startup we read the cache file and then do a
git rev-list to update it, which is fast because we exclude all commits
in the cache that have no children and commits reachable from them
(which amounts to everything in the cache).  If one of those commits
without children no longer exists, then git rev-list will give an error,
whereupon we throw away the cache and read in the whole tree again.

This gives a significant speedup in the startup time for gitk.

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

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