Skip syntax parsing when color is not being output.

[eth-p]

This pull request changes the InteractivePrinter to avoid unnecessarily parsing the language syntax when colored output is disabled. This was recognized by @Emilv2 in #2397.

Correctness

diff \
    <(./bat-master --color=never --decorations=always src/printer.rs -lrust --paging=never) \
    <(target/release/bat --color=never --decorations=always src/printer.rs) \
    && echo "No difference."
# No difference.

All tests pass, and there is no difference between outputs before and after these changes.

Benchmarks

When comparing it to 5a2a20a in the scenario outlined in #2397 (highlighting a recognized file with --color=never), the difference made by this PR is significant:

For the average use case where bat is outputting color for a single file, there does not appear to be a discernible difference.

Nor is there a discernible difference when printing multiple files:

[eth-p]

@sharkdp and @Enselic, a small question for context:

Over here, there's this code introduced in ce96df0 which returns from print_line when no HighlighterFromSet is available on the printer. Based on the commit message, this is meant to exit early in order to prevent the printer from displaying raw binary.

We also have this code just above it which returns early if the detected content type is binary.

Lastly, this code is what determines whether the highlighter_from_set is None. It will be set to None when the detected type is binary and -A is not provided.

In this pull request, I modified that last bit of code to also provide None when color output is disabled. Then, when highlighter_from_set is None, the newly-refactored highlight_regions_for_line function will return a fake (Style, &str) region with a dummy value for the Style. This shouldn't cause any problems, correct? I looked through the code, and nothing else seemed to rely on highlighter_from_set.

[sharkdp]

I agree with your analysis, it looks like we handled the binary case twice before, so this should be fine. Maybe do a quick test to make sure the new code (around EMPTY_SYNTECT_STYLE) will not be called in interactive-use binary-file scenarios?

[eth-p]

Seems to work as intended :)

No color + binary = just the header.
No color + binary + -A = the contents without color.
Color + binary + -A = the contents with color.
Color + binary = just the header with color.