Add support for merging baselines

From --help:
--update-baseline [file] Rewrite the existing baseline file with the
                         current set of warnings. If some warnings have
                         been fixed, this will delete them from the
                         baseline files. If a file is provided, the
                         updated baseline is written to the given file;
                         otherwise the original source baseline file is
                         updated.
--merge-baseline [file]  Like --update-baseline, but instead of always
                         replacing entries in the baseline, it will merge
                         the existing baseline with the new baseline.
                         This is useful if metalava runs multiple times
                         on the same source tree with different flags at
                         different times, such as occasionally with
                         --api-lint.

Test: Unit test added
Change-Id: If53a3df2d2a4b973de63babda981666c78d47133