Only lint files that have changed in ci_lint.sh

[foolip]

We are currently generating the whole manifest and linting all files in ci_lint.sh. This takes over 2 minutes on my desktop machine, and is probably much slower still on Travis.

Everything that needs to happen should be possible in <1 second for a typical PR:

1. Generate a manifest only for the modified files. (Assuming that only a file itself affects the classification.)
2. Run the lint only for modified files.

@bobholt linked to a discussion with @jgraham in http://logs.glob.uno/?c=w3%23testing&s=26+May+2017&e=26+May+2017

Do either of you know how long it currently takes on Travis in a typical case?

[jgraham]

The way I discussed doing this with gsnedders is to cache the lint status with the file hash and not check files that haven't changed for lints that are based purely on the file contents. Travis can store cached artifacts.

I'm not sure what generating a manifest only for certain files would look like, but my way means you don't need to ask the VCS which files changed.

[foolip]

The manifest is used to determine what lint steps are run, right? I was thinking pass some command line arguments so that the manifest only includes a few files, those that are needed.

Or, the manifest itself could be cached, we've done something like that in Chromium to get around the long build times for it.

[jgraham]

FWIW I have a branch that implements a simple version of this. Cuts the lint time pretty dramatically, but it depends on unmerged things in #6237

[jgraham]

Erm, make that #6421

[bobholt]

We merged this fix.