git p4: generate better error message for bad depot path

Depot paths must start with //.  Exit with a better explanation
when a bad depot path is supplied.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -3163,6 +3163,7 @@ class P4Clone(P4Sync):
         self.cloneExclude = ["/"+p for p in self.cloneExclude]
         for p in depotPaths:
             if not p.startswith("//"):
+                sys.stderr.write('Depot paths must start with "//": %s\n' % p)
                 return False
 
         if not self.cloneDestination:

t/t9800-git-p4-basic.sh

@@ -30,6 +30,11 @@ test_expect_success 'basic git p4 clone' '
 	)
 '
 
+test_expect_success 'depot typo error' '
+	test_must_fail git p4 clone --dest="$git" /depot 2>errs &&
+	grep "Depot paths must start with" errs
+'
+
 test_expect_success 'git p4 clone @all' '
 	git p4 clone --dest="$git" //depot@all &&
 	test_when_finished cleanup_git &&