Introduce RefDatabase#getTipsWithSha1 to list refs pointing to object

Introduce RefDatabase#getTipsWithSha1 to list refs pointing to object

Add resolveTipSha1, an inverse of exactRef(String ...), to RefDatabase
and provide a default implementation that runs in O(n) time where n is
the number of refs. For RefTable, provide an implementation that runs
in O(log(n)) time.

[ifrade@google.com: with tests in InMemoryRepositoryTest to exercise
 the reftable code path, too]

Change-Id: I2811ccd0339cdc1c74b42cce2ea003f07a2ce9e1
Signed-off-by: Patrick Hiesel <hiesel@google.com>
Signed-off-by: Ivan Frade <ifrade@google.com>