really write output file only if needed

[gdh1995]

This PR makes tsc only does really writing if an output file's content / BOM has been changed.

If one file is not updated, the log message will be:

• TSFILE (no changes): R:/Google/a.js

Intention

Although this slows down the compiler a little, I thinks it's a bad design to always write disk a lot everytime I change a single little file in my project.

For example, I'm migrating a JavaScript project to TypeScript, and I want to keep most of the built code same as the old, so I need to call tsc often. Here tsc --watch is useful, but it changes files' mtime frequently and make diff tools and zip do some unnecessary works.

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[HerringtonDarkholme]

Has #6937 already implemented incremental building?

If I have read the comment and code in #6937 correct, output will be skipped iff 1. the complied result changed and 2. someone other than compiler touched the file, regardless of modification.

Indeed reading file again and computing hash will skip some emission in certain case of 2. But I'm not sure whether it will outweigh the cost of reading/hashing relative to a stat call.

[gdh1995]

Updated secondly: My second commit makes watcher re-skip iff two fingerprints match, and the watcher will check content on the first time (on initing the fingerprint map).

Old content

@HerringtonDarkholme No, because #6937 only takes effects when --watch is enabled and it still write output files and update their mtime the first time a watching version tsc gets inited. That is, a tsc --watch will update all output files on initing, and then wait external / ts-file changes to select some files.

However, this PR makes --watch always use my new writeFileIfDiff to de-duplicate writing, and when --watch is not enabled, the check will still be done.

Updated: I made a mistake that --watch may not update the fingerprint's updated field, I'm writing a new commit.

Currently this PR has enabled all System instance to check diff, but if you want, we may disable this feature by a switch or on some special platforms.

[RyanCavanaugh]

What issue tracks this change?

In general the updating of an output file's timestamp is a good thing because it allows incremental build tools (e.g. make) to correctly determine which output files are out-of-date with respect to their inputs.

[mhegazy]

@gdh1995, thanks for the contribution. as a general policy, please file an issue first before submitting a change. I do not think we would want to take this change.

Most build systems do check time stamps before calling the compiler, e.g. gulp, MSBuild, etc.. so this is redundant, and all users have to pay this price regardless if they care about this feature or not.