exchange: use v2 bundles for modern compression engines (issue5506)

Previously, `hg bundle zstd` on a non-generaldelta repo would
attempt to use a v1 bundle. This would fail because zstd is not
supported on v1 bundles.

This patch changes the behavior to automatically use a v2 bundle
when the user explicitly requests a bundlespec that is a compression
engine not supported on v1. If the bundlespec is <engine>-v1, it is
still explicitly rejected because that request cannot be fulfilled.

mercurial/exchange.py

@@ -130,8 +130,12 b' def parsebundlespec(repo, spec, strict=T'
         if spec in util.compengines.supportedbundlenames:
             compression = spec
             version = 'v1'
+            # Generaldelta repos require v2.
             if 'generaldelta' in repo.requirements:
                 version = 'v2'
+            # Modern compression engines require v2.
+            if compression not in _bundlespecv1compengines:
+                version = 'v2'
         elif spec in _bundlespeccgversions:
             if spec == 'packed1':
                 compression = 'none'

tests/test-bundle-type.t

@@ -187,9 +187,7 b' Explicit request for zstd on non-general'
   $ hg --config format.usegeneraldelta=false init nogd
   $ hg -q -R nogd pull t1
   $ hg -R nogd bundle -a -t zstd nogd-zstd
-  abort: compression engine zstd is not supported on v1 bundles
-  (see 'hg help bundle' for supported values for --type)
-  [255]
+  1 changesets found
 
 zstd-v1 always fails