Support for resolving conflicts when using git-merge [INS-4638]

[yaoweiprc]

Show resolving conflicts modal when users merge another branch to current branch.
Users must commit uncommitted changes before merge.

• Update prompt-button component, now its onClick property can be an async function, and the button will show loading status when the promise is unsettled.
• Update merge method in git-vcs.ts, now it can catch MergeConflictErrors like what we did in git pull.
• Update mergeGitBranchAction method in git-actions.tsx, now it can be called directly instead of working as a data loader action.
• Update git-branches-modal.tsx
  • Add an errMsg line in each branch block to show error messages when operating branches. I think it's a better way than overlaying a warning popup on top of the current modal.
  • Add code to capture conflict errors and display a conflict resolution modal.
• Remove unused fetcher in git-staging-modal.tsx