t/test-lib: make sure Git has already been built

When tests were run without building git, they stopped with:

    .: 54: Can't open /path/to/git/source/t/../GIT-BUILD-OPTIONS

Move the check that makes sure that git has already been built from
t0000 to test-lib, so that any test will do so before it runs.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0000-basic.sh

@@ -18,16 +18,6 @@ swapping compression and hashing order, the person who is making the
 modification *should* take notice and update the test vectors here.
 '
 
-################################################################
-# It appears that people try to run tests without building...
-
-../git >/dev/null
-if test $? != 1
-then
-	echo >&2 'You do not seem to have built git yet.'
-	exit 1
-fi
-
 . ./test-lib.sh
 
 ################################################################

t/test-lib.sh

@@ -51,6 +51,15 @@ then
 fi
 GIT_BUILD_DIR="$TEST_DIRECTORY"/..
 
+################################################################
+# It appears that people try to run tests without building...
+"$GIT_BUILD_DIR/git" >/dev/null
+if test $? != 1
+then
+	echo >&2 'error: you do not seem to have built git yet.'
+	exit 1
+fi
+
 . "$GIT_BUILD_DIR"/GIT-BUILD-OPTIONS
 export PERL_PATH SHELL_PATH