Fix git commit generation

Not everybody builds VLC from the source tree
besides that the shell code was wrong : the return value of a pipe is the one of the LAST command in the pipe.

src/Makefile.am

@@ -380,7 +380,7 @@ SOURCES_libvlc_control = \
 misc/revision.c:
 	rm -f $@ $@.tmp
 	echo "/* AUTOGENERATED FILE - DO NOT EDIT */" > $@.tmp
-	REVISION="$$(LANG=C git-show-ref -s HEAD 2>/dev/null | cut -b -10 || echo exported)"; \
+	REVISION="$$((LANG=C git --git-dir=$(top_srcdir)/.git show-ref -s HEAD 2>/dev/null || echo exported) | cut -b -10)"; \
 	echo "const char psz_vlc_changeset[] = \"$$REVISION\";" >> $@.tmp
 	mv -f $@.tmp $@