If the way it works inconveniences you, it shouldn't be hard to make a simple wrapper - checkout a target branch, run a normal merge command, then checkout the original branch again. Since 2.22, the current branch name can be found with git branch --show-current, so that could be interpolated with backticks in a shell command, to save the original branch name for returning to it afterward.

Indeed! I might as well share the gmto script I've been using:

#!/usr/bin/env sh
#
# Checkout to the specified branch and merge the current one into it
# with a merge commit. Usage:
#
#   gmto <target-branch>
#
# Intended for a workflow like:
# 1. Develop feature in branch my-feature
# 2. When done, do gmto master
#
# This will checkout master and merge my-feature into it with a
# commit message.

set -e

# Figure out current branch
BRANCH=$(git branch --show-current)

# Checkout target branch
git checkout "$1"

# Merge
git merge --no-ff -e "$BRANCH"