commit: compute commit title for commit related files only

In cases were multiples files are staged for commit across packages, but
we run `pkgdev commit .` in one of them, it was adding and committing
only the current package, but computing the message based on both of
them.

Resolves: #122
Signed-off-by: Arthur Zamarin <arthurzam@gentoo.org>

src/pkgdev/scripts/pkgdev_commit.py

@@ -48,6 +48,9 @@ def parse_known_args(self, args=None, namespace=None):
         if namespace.edit:
             args.append("--edit")
         namespace.commit_args = args
+        namespace.git_args_paths = list(
+            filter(os.path.exists, (s for s in args if not s.startswith("-")))
+        )
         return namespace, []
 
 
@@ -575,6 +578,7 @@ def _generate_mapping(self):
             "--cached",
             "-z",
             "HEAD",
+            *self._options.git_args_paths,
             stdout=subprocess.PIPE,
         )