test-help: improve test coverage

Adds a test that extension commands are omitted from the default
command list.

Extends test coverage of commands.help_ by 1 line.

tests/test-help

@@ -48,4 +48,7 b' echo "helpext = $abspath" >> $HGRCPATH'
 echo %% test command with no help text
 hg help nohelp
 
+echo %% test that default list of commands omits extension commands
+hg help
+
 exit 0

tests/test-help.out

@@ -468,3 +468,77 b' hg nohelp'
 (no help text available)
 
 use "hg -v help nohelp" to show global options
+%% test that default list of commands omits extension commands
+Mercurial Distributed SCM
+
+list of commands:
+
+ add          add the specified files on the next commit
+ addremove    add all new files, delete all missing files
+ annotate     show changeset information by line for each file
+ archive      create an unversioned archive of a repository revision
+ backout      reverse effect of earlier changeset
+ bisect       subdivision search of changesets
+ branch       set or show the current branch name
+ branches     list repository named branches
+ bundle       create a changegroup file
+ cat          output the current or given revision of files
+ clone        make a copy of an existing repository
+ commit       commit the specified files or all outstanding changes
+ copy         mark files as copied for the next commit
+ diff         diff repository (or selected files)
+ export       dump the header and diffs for one or more changesets
+ forget       forget the specified files on the next commit
+ grep         search for a pattern in specified files and revisions
+ heads        show current repository heads or show branch heads
+ help         show help for a given topic or a help overview
+ identify     identify the working copy or specified revision
+ import       import an ordered set of patches
+ incoming     show new changesets found in source
+ init         create a new repository in the given directory
+ locate       locate files matching specific patterns
+ log          show revision history of entire repository or files
+ manifest     output the current or given revision of the project manifest
+ merge        merge working directory with another revision
+ outgoing     show changesets not found in destination
+ parents      show the parents of the working directory or revision
+ paths        show aliases for remote repositories
+ pull         pull changes from the specified source
+ push         push changes to the specified destination
+ recover      roll back an interrupted transaction
+ remove       remove the specified files on the next commit
+ rename       rename files; equivalent of copy + remove
+ resolve      retry file merges from a merge or update
+ revert       restore individual files or directories to an earlier state
+ rollback     roll back the last transaction
+ root         print the root (top) of the current working directory
+ serve        export the repository via HTTP
+ showconfig   show combined config settings from all hgrc files
+ status       show changed files in the working directory
+ summary      summarize working directory state
+ tag          add one or more tags for the current or given revision
+ tags         list repository tags
+ tip          show the tip revision
+ unbundle     apply one or more changegroup files
+ update       update working directory
+ verify       verify the integrity of the repository
+ version      output version and copyright information
+
+enabled extensions:
+
+ helpext  (no help text available)
+
+additional help topics:
+
+ config       Configuration Files
+ dates        Date Formats
+ patterns     File Name Patterns
+ environment  Environment Variables
+ revisions    Specifying Single Revisions
+ multirevs    Specifying Multiple Revisions
+ diffs        Diff Formats
+ templating   Template Usage
+ urls         URL Paths
+ extensions   Using additional features
+
+use "hg -v help" to show aliases and global options