deploy checkout unexpected revision when lib not include revision hash

This issue was introduced in https://github.com/ARMmbed/mbed-cli/pull/896.
The library in the cache can point to different branches every time.
On deploy/import/update, adding library _at latest revision in the current branch_, leads to unexpected revision checkout.
Example: As a result of Jenkins's job, library A in the cache points on branch `test`, Next job that will import library A without specific revision and expect `master` branch will end using `test` branch instead.