download/git: ensure we checkout to a clean state

Force the checkout to ignore and throw away any local changes. This
allows recovering from a previous partial checkout (e.g. killed by
the user, or by a CI job...)

git checkout -f has been supported since the inception of git, so we
can use it without any second thought.

Also do a forced-forced clean, to really get rid of all untracked stuff.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Tested-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

support/download/git

@@ -154,7 +154,11 @@ fi
 
 # Checkout the required changeset, so that we can update the required
 # submodules.
-_git checkout -q "'${cset}'"
+_git checkout -f -q "'${cset}'"
+
+# Get rid of now-untracked directories (in case a git operation was
+# interrupted in a previous run).
+_git clean -ffdx
 
 # Get date of commit to generate a reproducible archive.
 # %cD is RFC2822, so it's fully qualified, with TZ and all.