test-help: improve test coverage

Adds a test for handling ambiguous command help

Extends test coverage of commands.help_ by 4 lines.

tests/test-help

@@ -14,6 +14,10 b' echo %% test help option with version op'
 hg add -h --version | sed 's/[(]version [^)]*[)]/(version xxx)/'
 
 hg add --skjdfks
+
+echo %% test ambiguous command help
+hg help ad
+
 hg help diff
 hg help status
 hg -q help status

tests/test-help.out

@@ -294,6 +294,13 b' options:'
  -n --dry-run  do not perform actions, just print output
 
 use "hg -v help add" to show global options
+%% test ambiguous command help
+list of commands:
+
+ add         add the specified files on the next commit
+ addremove   add all new files, delete all missing files
+
+use "hg -v help ad" to show aliases and global options
 hg diff [OPTION]... [-r REV1 [-r REV2]] [FILE]...
 
 diff repository (or selected files)