Add configuration for git

- Add .config/git/config
  + Define aliases
  + Configure default behavior in various situations
  + Add user information with signing key
- Add .config/git/commit_msg_template.txt
- Add .config/git/ignore
- Integrate `git` aliases into the shell

.config/shell/aliases

@@ -5,3 +5,24 @@
 
 # Manage the bare `git` repository in ~/ holding the dotfiles
 alias dotfiles='git --git-dir=$XDG_DATA_HOME/dotfiles --work-tree=$HOME'
+
+
+# Integrate `git`
+if _command_exists git; then
+    alias g='git'
+
+    # Make all `git` aliases become shell aliases with a "g" prefix
+    for al in $(git aliases-internal); do
+        # Only "real" (i.e., short) aliases are created
+        [ ${#al} -lt 7 ] && eval "alias g$al='git $al'"
+    done
+
+    # Check if a "main" branch exists in place of a "master" branch
+    git_main_branch() {
+        if [[ -n "$(git branch --list main)" ]]; then
+           echo "main"
+        else
+           echo "master"
+        fi
+    }
+fi