gendoc: dispatch print document content by commandline arguments

Before this patch, gendoc.py only prints hg.1.gendoc.txt content.
This adds any content print function.

doc/Makefile

@@ -22,7 +22,7 hg.1.txt: hg.1.gendoc.txt
 	touch hg.1.txt
 
 hg.1.gendoc.txt: $(GENDOC)
-	${PYTHON} gendoc.py > $@.tmp
+	${PYTHON} gendoc.py hg.1.gendoc > $@.tmp
 	mv $@.tmp $@
 
 hgrc.5: ../mercurial/help/config.txt

doc/gendoc.py

@@ -1,3 +1,8
+"""usage: %s DOC ...
+
+where DOC is the name of a document
+"""
+
 import os, sys, textwrap
 # import from the live mercurial repo
 sys.path.insert(0, "..")
@@ -168,4 +173,11 def allextensionnames():
     return extensions.enabled().keys() + extensions.disabled().keys()
 
 if __name__ == "__main__":
+    doc = 'hg.1.gendoc'
+    if len(sys.argv) > 1:
+        doc = sys.argv[1]
+
+    if doc == 'hg.1.gendoc':
     showdoc(sys.stdout)
+    else:
+        showtopic(sys.stdout, sys.argv[1])