check-docs: do not expect guide pages to correspond to commands

When we want to see what commands are listed in `command-list.txt` but
not installed, we currently include lines that refer to guides, e.g.
`gitattributes` or `gitcli`.

Let's not include those lines, as they are not referring to commands.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -3078,6 +3078,7 @@ check-docs::
 	( \
 		sed -e '1,/^### command list/d' \
 		    -e '/^#/d' \
+		    -e '/guide$$/d' \
 		    -e 's/[ 	].*//' \
 		    -e 's/^/listed /' command-list.txt; \
 		$(MAKE) -C Documentation print-man1 | \