continue to work on fixed build

[László Nagy]

when the build breaks 'bear' record only those files which were compiled up until the break. but developers usually fix the break, and just build further (and not start a clean build) to see it goes well or not. this case the new files would overwrite the previous files.

before write a new compilation database, would read the old one and merge with the new set of files.

• this would solve the use case describe above
• this would deal with edit-compile-run cycles on normal development.
  • newly added files would be handled normally
  • deleted files could be detected by file existence check before write
• this would not detect compiler flag changes, and would cause double entries in the output.