ostree export: Add --prefix option

This lets you set a prefix for the resulting archive patsh.
Especially useful in combination with --subpath, for instance
--subpath=subdir --prefix=subdir to extract just subdir.

Closes: #265
Approved by: cgwalters

tests/test-export.sh

@@ -23,7 +23,7 @@ set -euo pipefail
 
 setup_test_repository "archive-z2"
 
-echo '1..3'
+echo '1..5'
 
 $OSTREE checkout test2 test2-co
 $OSTREE commit --no-xattrs -b test2-noxattrs -s "test2 without xattrs" --tree=dir=test2-co
@@ -47,7 +47,26 @@ assert_file_empty diff.txt
 
 echo 'ok export --subpath gnutar diff (no xattrs)'
 
-rm test2.tar test2-subpath.tar diff.txt t t2 -rf
+cd ${test_tmpdir}
+${OSTREE} 'export' test2-noxattrs --prefix=the-prefix/ -o test2-prefix.tar
+mkdir t3
+(cd t3 && tar xf ../test2-prefix.tar)
+${CMD_PREFIX} ostree --repo=repo diff --no-xattrs test2-noxattrs ./t3/the-prefix > diff.txt
+assert_file_empty diff.txt
+
+echo 'ok export --prefix gnutar diff (no xattrs)'
+
+cd ${test_tmpdir}
+${OSTREE} 'export' test2-noxattrs --subpath=baz --prefix=the-prefix/ -o test2-prefix-subpath.tar
+mkdir t4
+(cd t4 && tar xf ../test2-prefix-subpath.tar)
+${CMD_PREFIX} ostree --repo=repo diff --no-xattrs ./t4/the-prefix ./t/baz > diff.txt
+${CMD_PREFIX} ostree --repo=repo diff --no-xattrs test2-noxattrs ./t3/the-prefix > diff.txt
+assert_file_empty diff.txt
+
+echo 'ok export --prefix --subpath gnutar diff (no xattrs)'
+
+rm test2.tar test2-subpath.tar diff.txt t t2 t3 t4 -rf
 
 cd ${test_tmpdir}
 ${OSTREE} 'export' test2 -o test2.tar