download/git: add warning not to use our git cache

We really want the user not to use our git cache manually, or their
changes (committed or not) may eventually get lost.

So, add a warning file, not unlike the one we put in the target/
directory, to warn the user not to use the git tree.

Ideally, we would have carried this file in support/misc/, but the git
backend does not have access to it: the working directory is somewhere
unknown, and TOPDIR is not exported in the environment.

So, we have to carry it in-line in the backend instead.

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

@@ -43,6 +43,24 @@ _git() {
     eval GIT_DIR="${git_cache}/.git" ${GIT} "${@}"
 }
 
+# Create a warning file, that the user should not use the git cache.
+# It's ours. Our precious.
+cat <<-_EOF_ >"${dl_dir}/git.readme"
+	IMPORTANT NOTE!
+
+	The git tree located in this directory is for the exclusive use
+	by Buildroot, which uses it as a local cache to reduce bandwidth
+	usage.
+
+	Buildroot *will* trash any changes in that tree whenever it needs
+	to use it. Buildroot may even remove it in case it detects the
+	repository may have been damaged or corrupted.
+
+	Do *not* work in that directory; your changes will eventually get
+	lost. Do *not* even use it as a remote, or as the source for new
+	worktrees; your commits will eventually get lost.
+_EOF_
+
 # Initialise a repository in the git cache. If the repository already
 # existed, this is a noop, unless the repository was broken, in which
 # case this magically restores it to working conditions. In the latter