Description

When no kotlin files have been changed the list of patterns provided via stdin by the 'git diff' command is empty. Running ktlint then takes considerably longer as the entire project directory is traversed looking for kotlin files. But no such files will be found as the list of provided patterns is empty. So the project tree should only be traversed for a non-empty list of patterns.

Closes #2976

Checklist

Before submitting the PR, please check following (checks which are not relevant may be ignored):

• Commit message are well written. In addition to a short title, the commit message also explain why a change is made.
• At least one commit message contains a reference Closes #<xxx> or Fixes #<xxx> (replace<xxx> with issue number)
• Tests are added
• KtLint format has been applied on source code itself and violations are fixed
• PR title is short and clear (it is used as description in the release changelog)
• PR description added (background information)

Documentation is updated. See difference between snapshot and release documentation

• Snapshot documentation in case documentation is to be released together with a code change
• Release documentation in case documentation is related to a released version of ktlint and has to be published as soon as the change is merged to master