tools: add a script that checks submodules' commits

Sometimes to fix an issue we need to patch our submodule, update it
in tarantool and then patch tarantool using changes in submodule.

During development and review stages we have to create a branch (S)
in submodule and a branch (T) in tarantool that references head of
(S) branch.

When the fix is merged to mater it's very simple to make a mistake:
merge-and-push submodule's branch S and then tarantool's branch T.
It sounds obvious, but that's wrong: tarantool's master should
reference a commit from submodule's master, not a commit from
temporary developer's branch.

It's not hard to fix it but still a maintainer must always remember
that problem. In order to simplify their life it was decided to
create a script that is designed to check such thing before pushing
tarantool to origin/master.

Here is it, with simple usage:
./tools/check_push_master.sh && git push origin master

tools/check_push_master.sh

+#!/bin/bash
+
+# This script is recommended to run before pushing master to origin.
+# It fails (returns 1) if a problem detected, see output.
+exit_status=0
+
+# Check that submodules' commits are from their master branches.
+# The script checks all submodules, but fails only for the following list:
+our_submodules="src/lib/msgpuck:src/lib/small:test-run"
+
+function check_submodule() {
+    local submodule_path=$1
+    local commit=$2
+    git "--git-dir=$(git rev-parse --show-toplevel)/$submodule_path/.git" branch -r --contains $commit | egrep " origin/master$" > /dev/null
+    local commit_is_from_master=$?
+    [[ ":$our_submodules:" =~ ":$submodule_path:" ]]
+    local it_is_our_submodule=$?
+    if [[ $it_is_our_submodule -eq 0 ]] && [[ $commit_is_from_master -eq 1 ]]; then
+        echo "Submodule $submodule_path is set to commit $commit that is not in master branch!"
+        exit_status=1
+    fi
+}
+
+while read -r line; do
+    check_submodule $line
+done <<<"`git "--git-dir=$(git rev-parse --show-toplevel)/.git" ls-tree -r HEAD | egrep "^[0-9]+ commit" | awk '{print $4 " " $3}'`"
+
+# Done
+exit $exit_status
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