Makefile: add an explicit rule for building assembly output

In the kernel we have a rule for *.c -> *.s files exactly because
it's nice to be able to easily say "ok, what does that generate".

Here's a patch to add such a rule to git too, in case anybody is
interested. It makes it much simpler to just do

	make sha1_file.s

and look at the compiler-generated output that way, rather than having to
fire up gdb on the resulting binary.

(Add -fverbose-asm or something if you want to, it can make the result
even more readable)

[jc: add *.s to .gitignore]

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.gitignore

@@ -159,7 +159,7 @@ common-cmds.h
 *.deb
 git-core.spec
 *.exe
-*.[ao]
+*.[aos]
 *.py[co]
 config.mak
 autom4te.cache

Makefile

@@ -845,6 +845,8 @@ git$X git.spec \
 
 %.o: %.c GIT-CFLAGS
 	$(QUIET_CC)$(CC) -o $*.o -c $(ALL_CFLAGS) $<
+%.s: %.c GIT-CFLAGS
+	$(QUIET_CC)$(CC) -S $(ALL_CFLAGS) $<
 %.o: %.S
 	$(QUIET_CC)$(CC) -o $*.o -c $(ALL_CFLAGS) $<