Feat: Adds Ability to Merge MR (#147)

This adds the ability to merge an MR from within `gitlab.nvim` directly. If the reviewer is open, it'll be closed automatically. Users may configure whether they'd like to squash commits on the merge, as well as whether they'd like to delete the original source branch on a merge.

If squashing, users are prompted to provide an optional custom squash message for the squash commit.

cmd/merge.go

@@ -0,0 +1,71 @@
+package main
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+
+	"github.com/xanzy/go-gitlab"
+)
+
+type AcceptMergeRequestRequest struct {
+	Squash        bool   `json:"squash"`
+	SquashMessage string `json:"squash_message"`
+	DeleteBranch  bool   `json:"delete_branch"`
+}
+
+/* acceptAndMergeHandler merges a given merge request into the target branch */
+func (a *api) acceptAndMergeHandler(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "application/json")
+	w.Header().Set("Access-Control-Allow-Methods", http.MethodGet)
+	if r.Method != http.MethodPost {
+		handleError(w, InvalidRequestError{}, "Expected POST", http.StatusMethodNotAllowed)
+		return
+	}
+
+	body, err := io.ReadAll(r.Body)
+	if err != nil {
+		handleError(w, err, "Could not read request body", http.StatusBadRequest)
+		return
+	}
+
+	var acceptAndMergeRequest AcceptMergeRequestRequest
+	err = json.Unmarshal(body, &acceptAndMergeRequest)
+	if err != nil {
+		handleError(w, err, "Could not unmarshal request body", http.StatusBadRequest)
+		return
+	}
+
+	opts := gitlab.AcceptMergeRequestOptions{
+		Squash:                   &acceptAndMergeRequest.Squash,
+		ShouldRemoveSourceBranch: &acceptAndMergeRequest.DeleteBranch,
+	}
+
+	if acceptAndMergeRequest.SquashMessage != "" {
+		opts.SquashCommitMessage = &acceptAndMergeRequest.SquashMessage
+	}
+
+	_, res, err := a.client.AcceptMergeRequest(a.projectInfo.ProjectId, a.projectInfo.MergeId, &opts)
+
+	if err != nil {
+		handleError(w, err, "Could not merge MR", http.StatusInternalServerError)
+		return
+	}
+
+	if res.StatusCode >= 300 {
+		handleError(w, GenericError{endpoint: "/merge"}, "Could not merge MR", res.StatusCode)
+		return
+	}
+
+	response := SuccessResponse{
+		Status:  http.StatusOK,
+		Message: "MR merged successfully",
+	}
+
+	w.WriteHeader(http.StatusOK)
+
+	err = json.NewEncoder(w).Encode(response)
+	if err != nil {
+		handleError(w, err, "Could not encode response", http.StatusInternalServerError)
+	}
+}