Try to auto-detect revision in append mode if not specified

gitdch

@@ -10,6 +10,7 @@ import sys
 import textwrap
 
 import git
+from git.exc import GitCommandError
 
 VERSION = '0.0'
 DEFAULT_GIT_PATCH = './'
@@ -244,6 +245,30 @@ if not options.version:
         repo.git.describe('--always', '--tags')
     )
     log.info('Currrent version detected: %s', options.version)
+
+if options.output and options.append and not options.revision:
+    log.info(
+        'Append mode enabled but no revision specify, try to detect it from '
+        'last modification of the changelog file')
+    try:
+        last_change_commit = next(repo.iter_commits(paths=options.output))
+        # pylint: disable=consider-using-f-string
+        options.revision = '{0}..HEAD'.format(last_change_commit)
+        log.info(
+            'Last change commit of the output file is "%s": use revision "%s"',
+            last_change_commit, options.revision)
+    except StopIteration:
+        log.warning(
+            'Fail to auto-detect last change commit of changelog file: it '
+            'seem not tracked. Continue without revision.')
+    except GitCommandError:
+        log.warning(
+            "Fail to auto-detect last change commit of changelog file. May "
+            "be it's outside of the git repository. Continue without "
+            "revision.")
+        # Reset repo object of to avoid BrokenPipeError
+        repo = git.Repo(options.git_path)
+
 log.info('Generate changelog from git commits')
 
 versions = []