code-review-start: Add `get_merge_base` function

Function to get the merge base for the current branch. If none exists in
the database, default to `origin/master` if there's an `origin` remote,
or `master` if not.

Thanks to Serpiton
(https://codereview.stackexchange.com/users/61525/serpiton) on Stack
Overflow for describing how to get a default value using the `MAX()`
function:
https://codereview.stackexchange.com/questions/74192/select-first-value-if-exists-otherwise-select-default-value/74252#74252

Thanks to 'tig' (https://stackoverflow.com/users/96823/tig) on Stack
Overflow for the command to check if a given Git remote exists:
https://stackoverflow.com/questions/12170459/check-if-git-remote-exists-before-first-push/26843249#26843249

1 files changed, 25 insertions, 2 deletions

diff --git a/code-review-start b/code-review-start
index f4f8b37..0dc2265 100755
--- a/code-review-start
+++ b/code-review-start
@@ -52,9 +52,32 @@ function create_merge_base () {
 
 function create_merge_base_from_current () {
 	local base="$1"
-	local current_branch=$(git rev-parse --abbrev-ref HEAD)
 
-	create_merge_base "$current_branch" "$base"
+	create_merge_base "$(current_branch)" "$base"
+}
+
+function get_merge_base () {
+	local head="$(current_branch)"
+	local default_base='master'
+
+	if git config remote.origin.url > /dev/null; then
+		default_base='origin/master'
+	fi
+
+	sqlite3 "$DATABASE" <<-SQL
+		BEGIN TRANSACTION;
+
+		SELECT ifnull(max(base), '$default_base')
+		FROM merge_bases
+		WHERE head = '$head'
+		LIMIT 1;
+
+		COMMIT;
+	SQL
+}
+
+function current_branch () {
+	git rev-parse --abbrev-ref HEAD
 }