Fix MR Selection, Go Code Refactor (#358)

refactor: Refactors the Go codebase into a more modular and idiomatic approach
fix: require selection of specific MR when there are multiple targets for a given source branch
feat: Allows for the passing of Gitlab's filter options when choosing an MR, improves MR selection
feat: API to choose an MR from a list based on the provided username's involvement as an assignee/reviewer/author

This is a #MINOR release

cmd/app/revisions.go

@@ -0,0 +1,61 @@
+package app
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/xanzy/go-gitlab"
+)
+
+type RevisionsResponse struct {
+	SuccessResponse
+	Revisions []*gitlab.MergeRequestDiffVersion
+}
+
+type RevisionsGetter interface {
+	GetMergeRequestDiffVersions(pid interface{}, mergeRequest int, opt *gitlab.GetMergeRequestDiffVersionsOptions, options ...gitlab.RequestOptionFunc) ([]*gitlab.MergeRequestDiffVersion, *gitlab.Response, error)
+}
+
+type revisionsService struct {
+	data
+	client RevisionsGetter
+}
+
+/*
+revisionsHandler gets revision information about the current MR. This data is not used directly but is
+a precursor API call for other functionality
+*/
+func (a revisionsService) handler(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "application/json")
+	if r.Method != http.MethodGet {
+		w.Header().Set("Access-Control-Allow-Methods", http.MethodGet)
+		handleError(w, InvalidRequestError{}, "Expected GET", http.StatusMethodNotAllowed)
+		return
+	}
+
+	versionInfo, res, err := a.client.GetMergeRequestDiffVersions(a.projectInfo.ProjectId, a.projectInfo.MergeId, &gitlab.GetMergeRequestDiffVersionsOptions{})
+	if err != nil {
+		handleError(w, err, "Could not get diff version info", http.StatusInternalServerError)
+		return
+	}
+
+	if res.StatusCode >= 300 {
+		handleError(w, GenericError{endpoint: "/mr/revisions"}, "Could not get diff version info", res.StatusCode)
+		return
+	}
+
+	w.WriteHeader(http.StatusOK)
+	response := RevisionsResponse{
+		SuccessResponse: SuccessResponse{
+			Message: "Revisions fetched successfully",
+			Status:  http.StatusOK,
+		},
+		Revisions: versionInfo,
+	}
+
+	err = json.NewEncoder(w).Encode(response)
+	if err != nil {
+		handleError(w, err, "Could not encode response", http.StatusInternalServerError)
+	}
+
+}