test-help: improve test coverage

Adds a test for displaying a help topic.

Extends test coverage of commands.help_ by 5 lines.

tests/test-help

@@ -54,4 +54,7 b' hg help'
 echo %% test list of commands with command with no help text
 hg help helpext
 
+echo %% test a help topic
+hg help revs
+
 exit 0

tests/test-help.out

@@ -550,3 +550,33 b' list of commands:'
  nohelp   (no help text available)
 
 use "hg -v help helpext" to show aliases and global options
+%% test a help topic
+Specifying Single Revisions
+
+    Mercurial supports several ways to specify individual revisions.
+
+    A plain integer is treated as a revision number. Negative integers are
+    treated as sequential offsets from the tip, with -1 denoting the tip, -2
+    denoting the revision prior to the tip, and so forth.
+
+    A 40-digit hexadecimal string is treated as a unique revision identifier.
+
+    A hexadecimal string less than 40 characters long is treated as a unique
+    revision identifier and is referred to as a short-form identifier. A
+    short-form identifier is only valid if it is the prefix of exactly one
+    full-length identifier.
+
+    Any other string is treated as a tag or branch name. A tag name is a
+    symbolic name associated with a revision identifier. A branch name denotes
+    the tipmost revision of that branch. Tag and branch names must not contain
+    the ":" character.
+
+    The reserved name "tip" is a special tag that always identifies the most
+    recent revision.
+
+    The reserved name "null" indicates the null revision. This is the revision
+    of an empty repository, and the parent of revision 0.
+
+    The reserved name "." indicates the working directory parent. If no
+    working directory is checked out, it is equivalent to null. If an
+    uncommitted merge is in progress, "." is the revision of the first parent.