index-pack: add --progress-title option

Add a --progress-title option to index-pack, when data is piped into
index-pack its progress is a proxy for whatever's feeding it data.

This option will allow us to set a more relevant progress bar title in
"git bundle unbundle", and is also used in my "bundle-uri" RFC
patches[1] by a new caller in fetch-pack.c.

The code change in cmd_index_pack() won't handle
"--progress-title=xyz", only "--progress-title xyz", and the "(i+1)"
style (as opposed to "i + 1") is a bit odd.

Not using the "--long-option=value" style is inconsistent with
existing long options handled by cmd_index_pack(), but makes the code
that needs to call it better (two strvec_push(), instead of needing a
strvec_pushf()). Since the option is internal-only the inconsistency
shouldn't matter.

I'm copying the pattern to handle it as-is from the handling of the
existing "-o" option in the same function, see 9cf6d3357a (Add
git-index-pack utility, 2005-10-12) for its addition. That's a short
option, but the code to implement the two is the same in functionality
and style. Eventually we'd like to migrate all of this this to
parse_options(), which would make these differences in behavior go
away.

1. https://lore.kernel.org/git/RFC-cover-00.13-0000000000-20210805T150534Z-avarab@gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-index-pack.txt

@@ -82,6 +82,12 @@ OPTIONS
 --strict::
 	Die, if the pack contains broken objects or links.
 
+--progress-title::
+	For internal use only.
++
+Set the title of the progress bar. The title is "Receiving objects" by
+default and "Indexing objects" when `--stdin` is specified.
+
 --check-self-contained-and-connected::
 	Die if the pack contains broken links. For internal use only.