i18n: merge_file() deprecate positional arguments

[xclaesse]

They always have been ignored but it became an hard error with no
deprecation period in 0.60.0. Since it breaks some GNOME projects,
deprecate for now and keep it removed for 0.61.0.

Fixes #9441

[xclaesse]

@nirbheek this should go in 0.60 branch but not in master branch IMHO.

[codecov]

Codecov Report

Merging #9445 (022c035) into 0.60 (f64e5ce) will decrease coverage by 1.69%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             0.60    #9445      +/-   ##
==========================================
- Coverage   68.51%   66.82%   -1.70%     
==========================================
  Files         198      396     +198     
  Lines       42772    85442   +42670     
  Branches     8923    17657    +8734     
==========================================
+ Hits        29305    57094   +27789     
- Misses      11112    23660   +12548     
- Partials     2355     4688    +2333     

Impacted Files	Coverage Δ
mesonbuild/modules/i18n.py	76.78% <33.33%> (-1.60%)	⬇️
mesonbuild/scripts/vcstagger.py	87.50% <0.00%> (-4.17%)	⬇️
mesonbuild/minstall.py	67.88% <0.00%> (-0.77%)	⬇️
mesonbuild/modules/unstable_external_project.py	79.39% <0.00%> (-0.61%)	⬇️
mesonbuild/cmake/traceparser.py	70.10% <0.00%> (-0.22%)	⬇️
mesonbuild/cmake/interpreter.py	82.09% <0.00%> (ø)
compilers/cpp.py	38.95% <0.00%> (ø)
interpreterbase/__init__.py	100.00% <0.00%> (ø)
mintro.py	90.25% <0.00%> (ø)
mparser.py	92.62% <0.00%> (ø)
... and 223 more

---

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[lgtm-com]

This pull request introduces 1 alert when merging 999585b into 6ea4e21 - view on LGTM.com

new alerts:

• 1 for Unused import

[xclaesse]

@eli-schwartz while testing this, I noticed we can do a big simplification in merge_file() code too: #9457.

[nirbheek]

Is it just me or does this PR have too many commits?

[eli-schwartz]

It's developed against git master, and when I switched it to merge into the maintenance branch I created, I asked @xclaesse via IRC to rebase a single commit onto the maintenance branch.

This has not been done, though it did get rebased onto master again so now it includes even more commits from master. ;)

[nirbheek]

Okay, that makes sense. +1 from me to merge this into the 0.60 branch directly once Xavier is satisfied with it (and the rebase is done properly 😉)

[xclaesse]

Rebased on 0.60 now.