Support fetching without the --progress option (#1067)

Setting the `show-progress` option to false in the `with` section of the
workflow step will cause git fetch to run without `--progress`.

The motivation is to be able to suppress the noisy progress status
output which adds many hundreds of "remote: Counting objects: 85%
(386/453)" and similar lines in the workflow log.

This should be sufficient to resolve #894 and its older friends,
though the solution is different to the one proposed there because
it doesn't use the --quiet flag. IIUC git doesn't show the progress
status by default since the output is not a terminal, so that's why
removing the --progress option is all that's needed.

Adding the --quiet flag doesn't make a lot of difference once the
--progress flag is removed, and actually I think using --quiet would
suppress some other more useful output that would be better left
visible.

Signed-off-by: Simon Baird <sbaird@redhat.com>

README.md

@@ -91,6 +91,10 @@ When Git 2.18 or higher is not in your PATH, falls back to the REST API to downl
     # Default: false
     fetch-tags: ''
 
+    # Whether to show progress status output when fetching.
+    # Default: true
+    show-progress: ''
+
     # Whether to download Git-LFS files
     # Default: false
     lfs: ''