debuginstall: print compression engine support

Since compression engines may be provided by extensions and since
not all registered compression engines may be available to use,
it seems useful to provide a mechanism to see the state of known
compression engines.

This commit teaches `hg debuginstall` to print info on known and
available compression engines.

mercurial/commands.py

@@ -2579,6 +2579,16 b' def debuginstall(ui, **opts):'
         problems += 1
     fm.condwrite(err, 'extensionserror', " %s\n", err)
 
+    compengines = util.compengines._engines.values()
+    fm.write('compengines', _('checking registered compression engines (%s)\n'),
+             fm.formatlist(sorted(e.name() for e in compengines),
+                           name='compengine', fmt='%s', sep=', '))
+    fm.write('compenginesavail', _('checking available compression engines '
+                                   '(%s)\n'),
+             fm.formatlist(sorted(e.name() for e in compengines
+                                  if e.available()),
+                           name='compengine', fmt='%s', sep=', '))
+
     # templates
     p = templater.templatepaths()
     fm.write('templatedirs', 'checking templates (%s)...\n', ' '.join(p))

tests/test-install.t

@@ -11,6 +11,8 b' hg debuginstall'
   checking Mercurial custom build (*) (glob)
   checking module policy (*) (glob)
   checking installed modules (*mercurial)... (glob)
+  checking registered compression engines (*zlib*) (glob)
+  checking available compression engines (*zlib*) (glob)
   checking templates (*mercurial?templates)... (glob)
   checking default template (*mercurial?templates?map-cmdline.default) (glob)
   checking commit editor... (* -c "import sys; sys.exit(0)") (glob)
@@ -21,6 +23,8 b' hg debuginstall JSON'
   $ hg debuginstall -Tjson | sed 's|\\\\|\\|g'
   [
    {
+    "compengines": ["bz2", "bz2truncated", "none", "zlib"*], (glob)
+    "compenginesavail": ["bz2", "bz2truncated", "none", "zlib"*], (glob)
     "defaulttemplate": "*mercurial?templates?map-cmdline.default", (glob)
     "defaulttemplateerror": null,
     "defaulttemplatenotfound": "default",
@@ -58,6 +62,8 b' hg debuginstall with no username'
   checking Mercurial custom build (*) (glob)
   checking module policy (*) (glob)
   checking installed modules (*mercurial)... (glob)
+  checking registered compression engines (*zlib*) (glob)
+  checking available compression engines (*zlib*) (glob)
   checking templates (*mercurial?templates)... (glob)
   checking default template (*mercurial?templates?map-cmdline.default) (glob)
   checking commit editor... (* -c "import sys; sys.exit(0)") (glob)
@@ -85,6 +91,8 b' path variables are expanded (~ is the sa'
   checking Mercurial custom build (*) (glob)
   checking module policy (*) (glob)
   checking installed modules (*mercurial)... (glob)
+  checking registered compression engines (*zlib*) (glob)
+  checking available compression engines (*zlib*) (glob)
   checking templates (*mercurial?templates)... (glob)
   checking default template (*mercurial?templates?map-cmdline.default) (glob)
   checking commit editor... (* -c "import sys; sys.exit(0)") (glob)