make-pot: add subtract-and-merge flag

[swissspidy]

This PR adds a new --subtract-and-merge flag to the make-pot command, which can be used in combination with --subtract. When used, source code references from the generated POT file will instead be added to the POT file(s) passed to --subtract

Example:

# In foo-plugin/
wp i18n make-pot . foo-plugin.pot --subtract=exceptions.pot

# foo-plugin.pot will not contain any of the strings that are also in exceptions.pot
# exceptions.pot will now however contain all source code references for these strings

While I suggested adding dedicated commands for this feature a few years ago, in hindsight thats seems a bit overkill. It's quite the advanced configuration option that's probably only ever used for WordPress core, so just adding a flag feels like the better choice here.

Fixes #105

[swissspidy]

@ocean90 Would this solve the issue for dotorg?

[ocean90]

@ocean90 Would this solve the issue for dotorg?

As of https://meta.trac.wordpress.org/changeset/10063/ this is actually no issue anymore. My approach is also preserving any comments by using the mergeWith() method. Maybe this should be considered here as well?

[swissspidy]

Good suggestion with mergeWith() 👍

Are there chances that you would adopt this new --subtract-and-merge flag should it be added? If not, we might not want to bother implementing it.

[ocean90]

If if works the same as the current implementation, sure why not. Less code I have to maintain. 😅

[swissspidy]

It should work the same way now and is ready for review.