[Fix] install.sh: Force remote name of cloned repo to be 'origin'

The script assumes that the name of the remote is `origin`, but this
is not the case if the user has set `clone.defaultRemoteName` to
another value in the ~/.gitconfig (or elsewhere in the configuration).
Adding `-o origin` ensures that the remote will be called `origin`
regardless of the `clone.defaultRemoteName` setting.

Per PR #3341:
- The minimum Git version this should work with is v1.7.10. (This is not
  documented in the repo itself; it's just an implicit requirement.)
- The `--origin` option was added to `git clone` in commit 98a4fef3f2 which
  was released in v1.2.5. From the diff of that commit, the `-o` option was
  already available at that time. So this easily satisfies the above.
- A comment in #3341 indicates that `-o` was added in v1.1.0. I've not
  verified this, but we probably don't need to track that down since by the
  above we're already well within requirements.

install.sh

@@ -163,7 +163,7 @@ install_nvm_from_git() {
       }
     else
       # Cloning repo
-      command git clone "$(nvm_source)" --depth=1 "${INSTALL_DIR}" || {
+      command git clone -o origin "$(nvm_source)" --depth=1 "${INSTALL_DIR}" || {
         nvm_echo >&2 'Failed to clone nvm repo. Please report this!'
         exit 2
       }