btrfs-progs: ci: check for local copy of branch tar before downloading

The ci/test-build script unconditionally downloads the latest devel
snapshot. This is not practical for local development. Add a conditional
check for a file named devel.tar.gz, either it's missing or empty, then
download.

The empty file is also considered because this allows to use a docker
image that does not support conditional contents, so a stub file is a
fallback.

Signed-off-by: David Sterba <dsterba@suse.com>

ci/images/test-build

@@ -1,15 +1,22 @@
 #!/bin/sh
 # usage: $0 [branch name] [configure parameters]
 
-urlbase="https://github.com/kdave/btrfs-progs/archive/"
+urlbase="https://github.com/kdave/btrfs-progs/archive"
 branch=${1:-devel}
-url=${urlbase}${branch}.tar.gz
+fname=${branch}.tar.gz
+url="${urlbase}/${fname}"
 
 shift
 
 echo "btrfs-progs build test of branch ${branch}"
 cd /tmp
-wget "$url" -O ${branch}.tar.gz
-tar xf ${branch}.tar.gz
-cd btrfs-progs-${branch}
+if [ -f "$fname" -a -s "$fname" ]; then
+	echo "Found local file $fname, not downloading"
+else
+	echo "Missing or empty tar, downloading devel branch from git"
+	rm -- "$fname"
+	wget "$url" -O "$fname"
+fi
+tar xf "$fname"
+cd btrfs-progs-$branch
 ci/build-default "$@"