index-pack: document and test the --promisor option

The --promisor option of 'git index-pack' was created in 88e2f9e
(introduce fetch-object: fetch one promisor object, 2017-12-05) but was
untested. It is currently unused within the Git codebase, but that will
change in an upcoming change to 'git bundle unbundle' when there is a
filter capability.

For now, add documentation about the option and add a test to ensure it
is working as expected.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-index-pack.txt

@@ -122,6 +122,14 @@ This option cannot be used with --stdin.
 +
 include::object-format-disclaimer.txt[]
 
+--promisor[=<message>]::
+	Before committing the pack-index, create a .promisor file for this
+	pack. Particularly helpful when writing a promisor pack with --fix-thin
+	since the name of the pack is not final until the pack has been fully
+	written. If a `<message>` is provided, then that content will be
+	written to the .promisor file for future reference. See
+	link:technical/partial-clone.html[partial clone] for more information.
+
 NOTES
 -----
 

t/t5300-pack-object.sh

@@ -315,8 +315,10 @@ test_expect_success \
      git index-pack -o tmp.idx test-3.pack &&
      cmp tmp.idx test-1-${packname_1}.idx &&
 
-     git index-pack test-3.pack &&
+     git index-pack --promisor=message test-3.pack &&
      cmp test-3.idx test-1-${packname_1}.idx &&
+     echo message >expect &&
+     test_cmp expect test-3.promisor &&
 
      cat test-2-${packname_2}.pack >test-3.pack &&
      git index-pack -o tmp.idx test-2-${packname_2}.pack &&