Move Git config in bootstrap.sh into a function

SSH can be used to sign Git commits (a38644d), but the related settings
in `.gitconfig` are not compatible with all Git installations.

In GitHub Actions macOS workflows, a Git error message is seen after
`bootstrap.sh` clones the dotfiles repo and installs the dotfiles:

```text
error: unsupported value for gpg.format: ssh
```

This commit will consolidate the Git config steps in `bootstrap.sh` into
a function, and will add GPG configuration steps to avoid errors in CI.

bootstrap.sh

@@ -269,39 +269,44 @@ else
   log "Not macOS. Xcode CLT install and license check skipped."
 fi
 
-logn "Configuring Git:"
-if [ -n "$STRAP_GIT_NAME" ] && ! git config user.name >/dev/null; then
-  git config --global user.name "$STRAP_GIT_NAME"
-fi
-
-if [ -n "$STRAP_GIT_EMAIL" ] && ! git config user.email >/dev/null; then
-  git config --global user.email "$STRAP_GIT_EMAIL"
-fi
-
-if [ -n "$STRAP_GITHUB_USER" ] &&
-  [ "$(git config github.user)" != "$STRAP_GITHUB_USER" ]; then
-  git config --global github.user "$STRAP_GITHUB_USER"
-fi
-
-# Set up GitHub HTTPS credentials
-# shellcheck disable=SC2086
-if git credential-osxkeychain 2>&1 | grep $Q "git.credential-osxkeychain"; then
-  # Execute credential in case it's a wrapper script for credential-osxkeychain
-  if git "credential-$(git config --global credential.helper 2>/dev/null)" 2>&1 |
-    grep -v $Q "git.credential-osxkeychain"; then
-    git config --global credential.helper osxkeychain
+configure_git() {
+  logn "Configuring Git:"
+  if [ "$STRAP_CI" -gt 0 ]; then
+    git config --global commit.gpgsign false
+    git config --global gpg.format openpgp
   fi
-  if [ -n "$STRAP_GITHUB_USER" ] && [ -n "$STRAP_GITHUB_TOKEN" ]; then
-    PROTOCOL="protocol=https\\nhost=github.com"
-    printf "%s\\n" "$PROTOCOL" | git credential reject
-    printf "%s\\nusername=%s\\npassword=%s\\n" \
-      "$PROTOCOL" "$STRAP_GITHUB_USER" "$STRAP_GITHUB_TOKEN" |
-      git credential approve
-  else
-    log "Skipping Git credential setup."
+  if [ -n "$STRAP_GIT_NAME" ] && ! git config user.name >/dev/null; then
+    git config --global user.name "$STRAP_GIT_NAME"
   fi
-  logk
-fi
+  if [ -n "$STRAP_GIT_EMAIL" ] && ! git config user.email >/dev/null; then
+    git config --global user.email "$STRAP_GIT_EMAIL"
+  fi
+  if [ -n "$STRAP_GITHUB_USER" ] &&
+    [ "$(git config github.user)" != "$STRAP_GITHUB_USER" ]; then
+    git config --global github.user "$STRAP_GITHUB_USER"
+  fi
+  # Set up GitHub HTTPS credentials
+  # shellcheck disable=SC2086
+  if git credential-osxkeychain 2>&1 | grep $Q "git.credential-osxkeychain"; then
+    # Execute credential in case it's a wrapper script for credential-osxkeychain
+    if git "credential-$(git config --global credential.helper 2>/dev/null)" 2>&1 |
+      grep -v $Q "git.credential-osxkeychain"; then
+      git config --global credential.helper osxkeychain
+    fi
+    if [ -n "$STRAP_GITHUB_USER" ] && [ -n "$STRAP_GITHUB_TOKEN" ]; then
+      PROTOCOL="protocol=https\\nhost=github.com"
+      printf "%s\\n" "$PROTOCOL" | git credential reject
+      printf "%s\\nusername=%s\\npassword=%s\\n" \
+        "$PROTOCOL" "$STRAP_GITHUB_USER" "$STRAP_GITHUB_TOKEN" |
+        git credential approve
+    else
+      log "Skipping Git credential setup."
+    fi
+    logk
+  fi
+}
+
+configure_git
 
 # Check for and install any remaining software updates
 logn "Checking for software updates:"
@@ -342,6 +347,8 @@ log "Checking out $strap_dotfiles_branch_name in ~/.dotfiles."
 run_dotfile_scripts scripts/symlink.sh
 logk
 
+configure_git
+
 # shellcheck disable=SC2086
 install_homebrew() {
   logn "Installing Homebrew:"