hgmerge: fix diff+patch detection; cleanups
Details:
- actually show help message when no tools are found
- whitespace cleanup (stupid emacs)
- quote more variables
- simplify merge/diff3 error handling
transaction abort!
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abort: impossible time zone offset: 4444444
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abort: invalid date: '1\t15.1'
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abort: invalid date: 'foo bar'
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abort: invalid date: ' 1 4444'
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abort: date exceeds 32 bits: 111111111111
abort: No such file or directory: .../test/bar