tests: allow shell to be overridden

On some systems (e.g. Solaris), /bin/sh is not a POSIX shell.  Git
already provides suitable overrides in its config.mak.uname file and we
provide cgit.conf to allow the user to further change this.

The code for this is taken from Git's t/Makefile, meaning that we now
invoke the tests in the same way that Git does.

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

tests/Makefile

@@ -1,11 +1,15 @@
+include ../git/config.mak.uname
+-include ../cgit.conf
 
+SHELL_PATH ?= $(SHELL)
+SHELL_PATH_SQ = $(subst ','\'',$(SHELL_PATH))
 
 T = $(wildcard t[0-9][0-9][0-9][0-9]-*.sh)
 
 all: $(T)
 
 $(T):
-	@./$@ $(CGIT_TEST_OPTS)
+	@'$(SHELL_PATH_SQ)' $@ $(CGIT_TEST_OPTS)
 
 clean:
 	$(RM) -rf trash