submodule: try harder to fetch needed sha1 by direct fetching sha1

When reviewing a change that also updates a submodule in Gerrit, a
common review practice is to download and cherry-pick the patch
locally to test it. However when testing it locally, the 'git
submodule update' may fail fetching the correct submodule sha1 as
the corresponding commit in the submodule is not yet part of the
project history, but also just a proposed change.

If $sha1 was not part of the default fetch, we try to fetch the $sha1
directly. Some servers however do not support direct fetch by sha1,
which leads git-fetch to fail quickly. We can fail ourselves here as
the still missing sha1 would lead to a failure later in the checkout
stage anyway, so failing here is as good as we can get.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -591,6 +591,24 @@ cmd_deinit()
 	done
 }
 
+is_tip_reachable () (
+	clear_local_git_env
+	cd "$1" &&
+	rev=$(git rev-list -n 1 "$2" --not --all 2>/dev/null) &&
+	test -z "$rev"
+)
+
+fetch_in_submodule () (
+	clear_local_git_env
+	cd "$1" &&
+	case "$2" in
+	'')
+		git fetch ;;
+	*)
+		git fetch $(get_default_remote) "$2" ;;
+	esac
+)
+
 #
 # Update each submodule path to correct revision, using clone and checkout as needed
 #
@@ -745,10 +763,15 @@ Maybe you want to use 'update --init'?")"
 			then
 				# Run fetch only if $sha1 isn't present or it
 				# is not reachable from a ref.
-				(clear_local_git_env; cd "$sm_path" &&
+				is_tip_reachable "$sm_path" "$sha1" ||
-					( (rev=$(git rev-list -n 1 $sha1 --not --all 2>/dev/null) &&
+				fetch_in_submodule "$sm_path" ||
-					 test -z "$rev") || git-fetch)) ||
 				die "$(eval_gettext "Unable to fetch in submodule path '\$displaypath'")"
+
+				# Now we tried the usual fetch, but $sha1 may
+				# not be reachable from any of the refs
+				is_tip_reachable "$sm_path" "$sha1" ||
+				fetch_in_submodule "$sm_path" "$sha1" ||
+				die "$(eval_gettext "Fetched in submodule path '\$displaypath', but it did not contain $sha1. Direct fetching of that commit failed.")"
 			fi
 
 			# Is this something we just cloned?