bisect: error out when passing bad path parameters

As reported by Mark Lodato, "git bisect", when it was started with
path parameters that match no commit was kind of working without
taking account of path parameters and was reporting something like:

Bisecting: -1 revisions left to test after this (roughly 0 steps)

It is more correct and safer to just error out in this case, before
displaying the revisions left, so this patch does just that.

Note that this bug is very old, it exists at least since v1.5.5.
And it is possible to detect that case earlier in the bisect
algorithm, but it is not clear that it would be an improvement to
error out earlier, on the contrary it may change the behavior of
"git rev-list --bisect-all" for example, which is currently correct.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bisect.c

@@ -986,6 +986,12 @@ int bisect_next_all(const char *prefix)
 		exit(1);
 	}
 
+	if (!all) {
+		fprintf(stderr, "No testable commit found.\n"
+			"Maybe you started with bad path parameters?\n");
+		exit(4);
+	}
+
 	bisect_rev = revs.commits->item->object.sha1;
 	memcpy(bisect_rev_hex, sha1_to_hex(bisect_rev), 41);
 

t/t6030-bisect-porcelain.sh

@@ -567,6 +567,11 @@ test_expect_success 'skipping away from skipped commit' '
 	test "$para3" = "$PARA_HASH3"
 '
 
+test_expect_success 'erroring out when using bad path parameters' '
+	test_must_fail git bisect start $PARA_HASH7 $HASH1 -- foobar 2> error.txt &&
+	grep "bad path parameters" error.txt
+'
+
 #
 #
 test_done