GitGraph: added branch checking to rerouting

Hypothesised that working out which branch needed checking for
overlapping commits might be missing, so added that as a
nested ternary and passed result as new argument to rerouting
check.

If commits are found on the branch which will be getting the
curve (whichever branch is lower or more to the right of main
than the other, for now), then the arrow will be rerouted.

I may refactor in a follow-up commit and I think there's scope
to simplify the logic but this is a test for now.

packages/mermaid/src/diagrams/git/gitGraphRenderer.js

@@ -346,6 +346,7 @@ const drawCommits = (svg, commits, modifyGraph) => {
  *
  * @param {any} commitA
  * @param {any} commitB
+ * @param branchToGetCurve
  * @param allCommits
  * @returns {boolean}
  * If there are commits between
@@ -355,11 +356,11 @@ const drawCommits = (svg, commits, modifyGraph) => {
  * source branch is not main
  * return true
  */
-const shouldRerouteArrow = (commitA, commitB, allCommits) => {
-  const isOnSourceBranch = (x) => x.branch === commitA.branch;
+const shouldRerouteArrow = (commitA, commitB, branchToGetCurve, allCommits) => {
+  const isOnBranchToGetCurve = (x) => x.branch === branchToGetCurve;
   const isBetweenCommits = (x) => x.seq > commitA.seq && x.seq < commitB.seq;
   return Object.values(allCommits).some((commitX) => {
-    return isBetweenCommits(commitX) && isOnSourceBranch(commitX);
+    return isBetweenCommits(commitX) && isOnBranchToGetCurve(commitX);
   });
 };
 
@@ -396,11 +397,21 @@ const findLane = (y1, y2, depth = 0) => {
  * @param {any} allCommits
  */
 const drawArrow = (svg, commitA, commitB, allCommits) => {
-  const p1 = commitPos[commitA.id];
-  const p2 = commitPos[commitB.id];
-  const arrowNeedsRerouting = shouldRerouteArrow(commitA, commitB, allCommits);
+  const p1 = commitPos[commitA.id]; // arrowStart
+  const p2 = commitPos[commitB.id]; // arrowEnd
+  const branchToGetCurve =
+    dir === 'TB'
+      ? p1.x < p2.x
+        ? commitB.branch
+        : commitA.branch
+      : p1.y < p2.y
+      ? commitB.branch
+      : commitA.branch;
+  const arrowNeedsRerouting = shouldRerouteArrow(commitA, commitB, branchToGetCurve, allCommits);
   // log.debug('drawArrow', p1, p2, arrowNeedsReroute, commitA.id, commitB.id);
 
+  // Lower-right quadrant logic; top-left is 0,0
+
   let arc = '';
   let arc2 = '';
   let radius = 0;
@@ -418,30 +429,30 @@ const drawArrow = (svg, commitA, commitB, allCommits) => {
 
     if (dir === 'TB') {
       if (p1.x < p2.x) {
-        // Destination commit is on branch position left of source commit
-        // so render arrow leftward with colour of destination branch
+        // Source commit is on branch position left of destination commit
+        // so render arrow rightward with colour of destination branch
         colorClassNum = branchPos[commitB.branch].index;
         lineDef = `M ${p1.x} ${p1.y} L ${lineX - radius} ${p1.y} ${arc2} ${lineX} ${
           p1.y + offset
         } L ${lineX} ${p2.y - radius} ${arc} ${lineX + offset} ${p2.y} L ${p2.x} ${p2.y}`;
       } else {
-        // Destination commit is on branch position right of source commit
-        // so render arrow rightward with colour of source branch
+        // Source commit is on branch position right of destination commit
+        // so render arrow leftward with colour of source branch
         colorClassNum = branchPos[commitA.branch].index;
         lineDef = `M ${p1.x} ${p1.y} L ${lineX + radius} ${p1.y} ${arc} ${lineX} ${
           p1.y + offset
         } L ${lineX} ${p2.y - radius} ${arc2} ${lineX - offset} ${p2.y} L ${p2.x} ${p2.y}`;
       }
     } else {
       if (p1.y < p2.y) {
-        // Destination commit is on branch positioned below source commit
+        // Source commit is on branch positioned above destination commit
         // so render arrow downward with colour of destination branch
         colorClassNum = branchPos[commitB.branch].index;
         lineDef = `M ${p1.x} ${p1.y} L ${p1.x} ${lineY - radius} ${arc} ${
           p1.x + offset
         } ${lineY} L ${p2.x - radius} ${lineY} ${arc2} ${p2.x} ${lineY + offset} L ${p2.x} ${p2.y}`;
       } else {
-        // Destination commit is on branch positioned above source commit
+        // Source commit is on branch positioned below destination commit
         // so render arrow upward with colour of source branch
         colorClassNum = branchPos[commitA.branch].index;
         lineDef = `M ${p1.x} ${p1.y} L ${p1.x} ${lineY + radius} ${arc2} ${
@@ -488,10 +499,8 @@ const drawArrow = (svg, commitA, commitB, allCommits) => {
         arc = 'A 20 20, 0, 0, 0,';
         radius = 20;
         offset = 20;
-
-        // Figure out the color of the arrow,arrows going down take the color from the destination branch
+        // Arrows going up take the color from the target branch
         colorClassNum = branchPos[commitB.branch].index;
-
         lineDef = `M ${p1.x} ${p1.y} L ${p1.x} ${p2.y - radius} ${arc} ${p1.x + offset} ${p2.y} L ${
           p2.x
         } ${p2.y}`;
@@ -500,7 +509,6 @@ const drawArrow = (svg, commitA, commitB, allCommits) => {
         arc = 'A 20 20, 0, 0, 0,';
         radius = 20;
         offset = 20;
-
         // Arrows going up take the color from the source branch
         colorClassNum = branchPos[commitA.branch].index;
         lineDef = `M ${p1.x} ${p1.y} L ${p2.x - radius} ${p1.y} ${arc} ${p2.x} ${p1.y - offset} L ${