update to got 0.18
- always show the input path when realpath(3) returns an error
- preserve log message when an error occurs during commit, tag, or import
- ignore empty files in refs directory instead of throwing errors
- show file mode differences (executable bit) when diffing in-repository trees
- fix handling of executable-bit changes during update, commit, and status
- improve tog's responsiveness after loading hundreds of thousands of commits
- fix tog performance bug which made searching through commits very slow
- add 'got integrate' command for merging a branch back to its parent branch
- show content from the merge-base file in 3-way merge conflicts, if available
- improved indication of 3-way merge inputs in conflict marker labels
- handle read errors in got_packidx_init_hdr (found by jj@ with scan-build)
- remove worklist code; it was only used to unlink files already unlinked
- show bad reference names in error messages to make such errors more clear
- add -w (ignore whitespace) option to 'got diff'
- improved coding style and error handling in diff3 code
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