Fix bufsz decreasing in get_line()

Used to use at most bufsz/2 characters in the buffer.
Return NULL on too small buffer for get_line()

This fixes creating of eternal loops by function's callers.
Add 5 unit-tests for the get_line() function
Increase max preview line width four times

So very-very-long lines are not truncated.
Fix hanging on previewing files with long lines

Requirement: enabled code highlighting.
Merge branch 'fix-hang-on-file-preview'