gitk: Speed up resolution of short SHA1 ids

gitk: Speed up resolution of short SHA1 ids

On large repositories such as the Linux kernel, it can take quite a
noticeable time (several seconds) for gitk to resolve short SHA1 IDs
to their long form.  This speeds up the process by maintaining lists
of IDs indexed by the first 4 characters of the SHA1 ID, speeding up
the search by a factor of 65536 on large repositories.

Signed-off-by: Paul Mackerras <redacted>