Run golint for each file that has changed

[varunpatro]

Also tracked at #529.

If I change one file main.go, then the GoLint hook runs golint main.go which is desirable and correct.

However, when I change files that belong to multiple packages such as:

1. main.go
2. lib/util.go

then the GoLint hook runs golint main.go lib/util.go which will always return the error (regardless of the presence of any lint errors)

lib/util.go is in package lib, not main

This is because golint can only be used in the following way (from golint --help:

Usage of golint:
golint [flags] # runs on package in current directory
golint [flags] [packages]
golint [flags] [directories] # where a '/...' suffix includes all sub-directories
golint [flags] [files] # all must belong to a single package

This means golint shouldn't be passed files that belong to different packages.

This commit fixes this behaviour by running golint separately on each file that has changed. And the concatenating all their outputs.

[pkopac]

I've just hit this bug as well.

[trotzig]

Thanks for the PR, and sorry for the long wait. This looks good. My only worry is performance, since now we're starting a new golint process for every file. We should have a plan for mitigating that if people start complaining after we merge and release this. My skills with go are very limited, is there a way to detect when something is a new package vs when it's just a subfolder? If so, we can split the command up based on package folders instead of each file.