Makefile: remove "cscope.out", not "cscope*" in cscope.out target

Before we generate a "cscope.out" file, remove that file explicitly,
and not everything matching "cscope*". This doesn't change any
behavior of the Makefile in practice, but makes this rule less
confusing, and consistent with other similar rules.

The cscope target was added in a2a9150bf0 (makefile: Add a cscope
target, 2007-10-06). It has always referred to cscope* instead of to
cscope.out in .gitignore and the "clean" target, even though we only
ever generated a cscope.out file.

This was seemingly done to aid use-cases where someone invoked cscope
with the "-q" flag, which would make it create a "cscope.in.out" and
"cscope.po.out" files in addition to "cscope.out".

But us removing those files we never generated is confusing, so let's
only remove the file we need to, furthermore let's use the "-f" flag
to explicitly name the cscope.out file, even though it's the default
if not "-f" argument is supplied.

It is somewhat inconsistent to change from the glob here but not in
the "clean" rule and .gitignore, an earlier version of this change
updated those as well, but see [1][2] for why they were kept.

1. https://lore.kernel.org/git/87k0lit57x.fsf@evledraar.gmail.com/
2. https://lore.kernel.org/git/87im0kn983.fsf@evledraar.gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -2740,8 +2740,8 @@ tags: $(FOUND_SOURCE_FILES)
 	mv tags+ tags
 
 cscope.out: $(FOUND_SOURCE_FILES)
-	$(QUIET_GEN)$(RM) cscope* && \
-	echo $(FOUND_SOURCE_FILES) | xargs cscope -b
+	$(QUIET_GEN)$(RM) $@ && \
+	echo $(FOUND_SOURCE_FILES) | xargs cscope -f$@ -b
 
 .PHONY: cscope
 cscope: cscope.out