Add Mercurial dist support

[QuLogic]

I was going to copy all the git code but as far as I can tell, hg archive does everything we need except for not supporting .xz files (which can easily be approximated out of Python stdlib.)

As far as I can tell, you can't clone without subrepos, so we don't need the extra code to bundle them, and Mercurial has a flag to include subrepos (-S) in archives anyway.

[codecov-io]

Codecov Report

Merging #1920 into master will decrease coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1920      +/-   ##
==========================================
- Coverage   63.51%   63.46%   -0.05%     
==========================================
  Files          58       58              
  Lines       16816    16828      +12     
  Branches     3475     3476       +1     
==========================================
  Hits        10680    10680              
- Misses       5067     5079      +12     
  Partials     1069     1069

Impacted Files	Coverage Δ
mesonbuild/scripts/dist.py	0% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f75d394...eff273a. Read the comment docs.

[jpakkane]

This code needs to check if the hg binary is installed and throw a skip exception if it's not.

Also, since we are in release freeze, new features will no longer be added. This will go in the next release once trunk opens.

[QuLogic]

It already checks for hg; that's how Travis passes. As I pointed out in IRC though, it doesn't check for vs* backend and neither does the git dist test, yet somehow the git dist test passes, which confuses me.

new features will no longer be added.

That depends on whether you consider this a new feature or just completion of an existing new feature. :P

[QuLogic]

I added a skip when not running with Ninja, but I still don't understand how the git one manages to pass on other backends.