MergedReftableTest#scanDuplicates tests whether we can write duplicate
keys in a merged reftable. Apparently, the first key appearing should
get precedence, and this works because the sort() algorithm on ordered
collections is stable.

This is potentially confusing behavior, because you can write data
into the table that cannot be retrieved (Merged table can only have
one entry per key), and the APIs such as exactRef() only return a
single value.

Make this consistent with behavior introduced in I04f55c481 "reftable:
enforce ordering for ref and log writes" by considering a duplicate key
in sortAndWriteRefs as a fatal runtime error.

Change-Id: I1eedd18f028180069f78c5c467169dcfe1521157
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>

By using ${min_update}-${max_update} as file name template, we
guarantee that each file has a unique name.
This allows data from open files to be cached across reloads of the
stack.

This is in anticipation of Change I1837f268e 
("file: implement FileReftableDatabase"), which is the first
implementation of reftable on a filesystem.

Change-Id: I7ef0610eb60c494165382d0c372afcf41f074393
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>

Change-Id: I0fe7d28a772b1ee9eefd9a38bff5e08a8559988f
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>

Add an update_index to every reference in a reftable, storing the
exact transaction that last modified the reference.  This is necessary
to fix some merge race conditions.

Consider updates at T1, T3 are present in two reftables.  Compacting
these will create a table with range [T1,T3].  If T2 arrives during
or after the compaction its impossible for readers to know how to
merge the [T1,T3] table with the T2 table.

With an explicit update_index per reference, MergedReftable is able to
individually sort each reference, merging individual entries at T3
from [T1,T3] ahead of identically named entries appearing in T2.

Change-Id: Ie4065d4176a5a0207dcab9696ae05d086e042140

Some repositories contain a lot of references (e.g. android at 866k,
rails at 31k). The reftable format provides:

- Near constant time lookup for any single reference, even when the
  repository is cold and not in process or kernel cache.
- Near constant time verification a SHA-1 is referred to by at least
  one reference (for allow-tip-sha1-in-want).
- Efficient lookup of an entire namespace, such as `refs/tags/`.
- Support atomic push `O(size_of_update)` operations.
- Combine reflog storage with ref storage.

Change-Id: I29d0ff1eee475845660ac9173413e1407adcfbf2