fast-import: add (non-)relative-marks feature

After specifying 'feature relative-marks' the paths specified with
'feature import-marks' and 'feature export-marks' are relative to an
internal directory in the current repository.

In git-fast-import this means that the paths are relative to the
'.git/info/fast-import' directory. However, other importers may use a
different location.

Add 'feature non-relative-marks' to disable this behavior, this way
it is possible to, for example, specify the import-marks location as
relative, and the export-marks location as non-relative.

Also add tests to verify this behavior.

Cc: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t9300-fast-import.sh b/t/t9300-fast-import.sh
index ba92775..a1b8c2b 100755
--- a/t/t9300-fast-import.sh
+++ b/t/t9300-fast-import.sh
@@ -1346,6 +1346,31 @@
     test_cmp marks.out combined.marks
 '
 
+cat >input <<EOF
+feature relative-marks
+feature import-marks=relative.in
+feature export-marks=relative.out
+EOF
+
+test_expect_success 'R: feature relative-marks should be honoured' '
+    mkdir -p .git/info/fast-import/ &&
+    cp marks.new .git/info/fast-import/relative.in &&
+    git fast-import <input &&
+    test_cmp marks.new .git/info/fast-import/relative.out
+'
+
+cat >input <<EOF
+feature relative-marks
+feature import-marks=relative.in
+feature no-relative-marks
+feature export-marks=non-relative.out
+EOF
+
+test_expect_success 'R: feature no-relative-marks should be honoured' '
+    git fast-import <input &&
+    test_cmp marks.new non-relative.out
+'
+
 cat >input << EOF
 option git quiet
 blob