Support merge of a branch to its upstream

**Is your feature request related to a problem? Please describe.**
When using Git from a command line, especially for downloading updates for some software I otherwise do not contribute to (that is, my master is always to be fast-forwarded, never divergent), my workflow is:
```
git fetch
# occasionally—though by no means often—check the fetched commits
git merge
```
Of course after the first (or optional second) step one could call `git rebase` with the same effect (again, in the case of no divergence), however I prefer `git merge` as it offers more information on the amount of changes as well as the names of changed files, whereas rebase only informs about success. For example:
```
[master] $ git merge
Updating d159b28dc..143d4760f
Fast-forward
.devcontainer/Dockerfile | 2 --
.github/workflows/ci.yml | 1 +
.gitignore | 1 -
.vscode/settings.json | 2 +-
Makefile | 2 +-
scripts/golangci-lint-shim.sh | 8 ++++++++
scripts/lint.sh | 12 ------------
7 files changed, 11 insertions(+), 17 deletions(-)
create mode 100755 scripts/golangci-lint-shim.sh
delete mode 100755 scripts/lint.sh
```
Lazygit however does not offer merge in this situation, in fact it pretends—or I have been unable to find the correct key—that no merge is possible. That's both wrong and irritating: auxiliary tools shouldn't lie about underlying software.

**Describe the solution you'd like**
I'd like to have option to merge a branch to its already fetched upstream, displaying the same info as when called from command line.