check-docs: really look at the documented commands again

As part of the `check-docs` target, we verify that commands that are
documented are actually in the current list of commands to be built.

However, this logic broke in 5fafce0b78 (check-docs: get documented
command list from Makefile, 2012-08-08), when we tried to make the logic
safer by not looking at the files in the worktree, but at the list of
files to be generated in `Documentation/Makefile`. While this was the
right thing to do, it failed to accommodate for the fact that `make -C
Documentation/ print-man1`, unlike `ls Documentation/*.txt`, would *not*
print lines starting with the prefix `Documentation/`.

At long last, let's fix this.

Note: This went undetected due to a funny side effect of the
`ALL_PROGRAMS` variable starting with a space. That space, together with
the extra space we inserted before `$(ALL_PROGRAMS)` in the

	case " $(ALL_PROGRAMS)" in
	*" $$cmd ")
		[...]

construct, is responsible that this case arm is used when `cmd` is empty
(which was clearly not intended to be the case).

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

Makefile

@@ -3082,7 +3082,7 @@ check-docs::
 		    -e 's/^/listed /' command-list.txt; \
 		$(MAKE) -C Documentation print-man1 | \
 		grep '\.txt$$' | \
-		sed -e 's|Documentation/|documented |' \
+		sed -e 's|^|documented |' \
 		    -e 's/\.txt//'; \
 	) | while read how cmd; \
 	do \