builtin-remote: make rm operation safer in mirrored repository

"git remote rm <repo>" happily removes non-remote refs and their reflogs.
This may be okay if the repository truely is a mirror, but if the user
had done "git remote add --mirror <repo>" by accident and was just
undoing their mistake, then they are left in a situation that is
difficult to recover from.

After this commit, "git remote rm" skips over non-remote refs. The user
is advised on how remove branches using "git branch -d", which itself
has nice safety checks wrt to branch removal lacking from "git remote rm".
Non-remote non-branch refs are skipped silently.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t5505-remote.sh b/t/t5505-remote.sh
index 0103e1a..aadf3e9 100755
--- a/t/t5505-remote.sh
+++ b/t/t5505-remote.sh
@@ -107,6 +107,32 @@
 )
 '
 
+test_expect_success 'remove remote protects non-remote branches' '
+(
+	cd test &&
+	(cat >expect1 <<EOF
+Note: A non-remote branch was not removed; to delete it, use:
+  git branch -d master
+EOF
+    cat >expect2 <<EOF
+Note: Non-remote branches were not removed; to delete them, use:
+  git branch -d foobranch
+  git branch -d master
+EOF
+) &&
+	git tag footag
+	git config --add remote.oops.fetch "+refs/*:refs/*" &&
+	git remote rm oops 2>actual1 &&
+	git branch foobranch &&
+	git config --add remote.oops.fetch "+refs/*:refs/*" &&
+	git remote rm oops 2>actual2 &&
+	git branch -d foobranch &&
+	git tag -d footag &&
+	test_cmp expect1 actual1 &&
+	test_cmp expect2 actual2
+)
+'
+
 cat > test/expect << EOF
 * remote origin
   URL: $(pwd)/one