unit tests for PGLog::merge_log

The tests covers 100% of the LOC of merge_log. It is broken down
in 7 cases to enumerate all the situations it must address. Each case
is isolated in a independant code block where the conditions are
reproduced. Where possible and sensible to read, a code block covers
as much lines as possible. For instance:

  The log entry (1,3) deletes the object x9 but the olog entry (2,3)
  modifies it and is authoritative : the log entry (1,3) is divergent.

is the only test case covering a dozen "if" statements and half a
dozen "while/for" loops. It covers all the lines but it would be
useful to create others scenarii in the future.

Each test is made of a comment describing the test case, the
definition of the data structures to create the desired conditons, a
sequence of EXPECT_* checking that they are met, a single call to
merge_log and another sequence of EXPECT_* ( ordered to be easy to
compare with the first sequence ) checking all the desired side
effects.

The TestPGLog.cc file was untabified to improve the display of ascii
art when it is output as part of a diff.

http://tracker.ceph.com/issues/5213 refs #5213

Signed-off-by: Loic Dachary <loic@dachary.org>