tests: use Git's test framework

This allows tests to run in parallel as well as letting us use "prove"
or another TAP harness to run the tests.

Git's test framework requires Git to be fully built before letting any
tests run, so add a new target to the top-level Makefile which builds
all of Git instead of just libgit.a and make the "test" target depend on
that.

Signed-off-by: John Keeping <john@keeping.me.uk>

Makefile

@@ -66,7 +66,10 @@ all:: cgit
 cgit:
 	$(QUIET_SUBDIR0)git $(QUIET_SUBDIR1) -f ../cgit.mk ../cgit NO_CURL=1
 
-test: all
+git:
+	$(QUIET_SUBDIR0)git $(QUIET_SUBDIR1) NO_CURL=1
+
+test: all git
 	$(QUIET_SUBDIR0)tests $(QUIET_SUBDIR1) all
 
 install: all
@@ -145,7 +148,7 @@ get-git:
 tags:
 	$(QUIET_TAGS)find . -name '*.[ch]' | xargs ctags
 
-.PHONY: all cgit get-git
+.PHONY: all cgit git get-git
 .PHONY: clean clean-doc cleanall
 .PHONY: doc doc-html doc-man doc-pdf
 .PHONY: install install-doc install-html install-man install-pdf