Makefile: make perf tests optional for profile build

The perf tests need a repository to operate on; if none is
defined, we fall back to the repository containing our build
directory.  That fails, though, for an exported tarball of
git.git, which has no repository.

Since 5d7fd6d we run the perf tests as part of "make
profile". Therefore "make profile" fails out of the box on
released tarballs of v2.1.0.

We can fix this by making the perf tests optional; if they
are skipped, we still run the regular test suite, which
should give a lot of profile data (and is what we used to do
prior to 5d7fd6d anyway).

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1649,7 +1649,11 @@ endif
 profile:: profile-clean
 	$(MAKE) PROFILE=GEN all
 	$(MAKE) PROFILE=GEN -j1 test
-	$(MAKE) PROFILE=GEN -j1 perf
+	@if test -n "$$GIT_PERF_REPO" || test -d .git; then \
+		$(MAKE) PROFILE=GEN -j1 perf; \
+	else \
+		echo "Skipping profile of perf tests..."; \
+	fi
 	$(MAKE) PROFILE=USE all
 
 profile-fast: profile-clean