fix(import): Better handling of "Patch is empty"

[TimShilov]

Description

Removed check for hard-coded "Patch is empty." git message in favor of git diff COMMIT^! check.
Fixes #2351 , fixes #2587

Motivation and Context

Command lerna import --flatten <path/to/repo> fails if LANG environment variable set to ru_RU.UTF-8 (default value on macOS in Russia).
Example output:

lerna ERR! import Rolling back to previous HEAD (commit fff5b779833c57820df2f76c812fd0e89641de07)
lerna ERR! EIMPORT Failed to apply commit ef18c6c.
lerna ERR! EIMPORT Command failed: git am -3 --keep-non-patch
lerna ERR! EIMPORT Патч пустой.
lerna ERR! EIMPORT Когда вы устраните эту проблему, запустите «git am --continue».
lerna ERR! EIMPORT Если вы хотите пропустить этот патч, то запустите «git am --skip».
lerna ERR! EIMPORT Чтобы вернуться на предыдущую ветку и остановить применение изменений, запустите «git am --abort».
lerna ERR! EIMPORT 
lerna ERR! EIMPORT 
lerna ERR! EIMPORT You may try again with --flatten to import flat history.

How Has This Been Tested?

Updated command's code and successfully imported repo with multiple locales including: en_US.UTF-8, ru_RU.UTF-8, en_IN.utf8, es_ES.UTF-8 and de_CH.ISO8859-1.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[evocateur]

Given that the import command is a list of hacks to begin with, is there any other way we can know the "emptiness" of the commit? I'm reticent to add a list of hard-coded localized strings, but I understand hard-coded English isn't a better solution.

[TimShilov]

@evocateur Hi!
Sorry for pinging you but I really need that fix.
I fixed your change request. Is there anything I can/need to do to get this merged?

[evocateur]

Thanks!