yarn config rewrites

[izenk]

bug
Ubuntu 16.04 x64
yarn 1.15.2

What is the current behavior?
Yarn config command rewrites relative paths in some cases to absolute paths

To reproduce in my case

cd to project directory
yarn config set yarn-offline-mirror vendor/npm_cache
look at /home/<your_user>/.yarnrc and find that

# yarn lockfile v1
yarn-offline-mirror vendor/npm_cache

then run
yarn config set yarn-offline-mirror-pruning true
and check /home/<your_user>/.yarnrc again, you will find

# yarn lockfile v1
yarn-offline-mirror "/home/app/vendor/npm_cache"
yarn-offline-mirror-pruning true

Relative path of offline-mirror changed to absolute path.

If you run again

yarn config set yarn-offline-mirror vendor/npm_cache

It will be change to relative again in .yarnrc

If you run first
yarn config set yarn-offline-mirror-pruning true
and then

yarn config set yarn-offline-mirror vendor/npm_cache

Everything is as expected

# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.
# yarn lockfile v1

yarn-offline-mirror "vendor/npm_cache"
yarn-offline-mirror-pruning true

What is the expected behavior?
The offline-mirror path should stay the same as was set previously.

[izenk]

One more example of incorrect behavior.

If module-path option is added to yarnrc, then 'yarn bin' works incorrectly

app@8613468dd8e9:~/core$ cat .yarnrc && yarn bin
--modules-folder vendor/node_modules
warning package.json: No license field
/home/app/core/home/app/core/vendor/node_modules/.bin

As seen, for some reason <app_path> (which is '/home/app/core' in my case) is duplicated, what leads to incorrect behavior.