gc doc: mention the commit-graph in the intro

Explicitly mention in the intro that we may be writing supplemental data structures such as the commit-graph during "gc", i.e. to call out the "optimize" part of what this command does, it doesn't just "collect garbage" as the "gc" name might imply.

Past changes have updated the intro to reflect new commands, such as mentioning "worktree" in b586a96a39 ("gc.txt: more details about what gc does", 2018-03-15). So let's elaborate on what was added in d5d5d7b641 ("gc: automatically write commit-graph files", 2018-06-27).

See also
https://public-inbox.org/git/[email protected]/ (follow-up replies) for an on-list discussion about what "gc" does.

---

In light of the linked thread let's see how controversial this is as a stand-alone :)

 Documentation/git-gc.txt | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

index f5bc98ccb3..c20ee6c789 100644
@@ -17,7 +17,8 @@ Runs a number of housekeeping tasks within the current repository,
 such as compressing file revisions (to reduce disk space and increase
 performance), removing unreachable objects which may have been
 created from prior invocations of 'git add', packing refs, pruning
-reflog, rerere metadata or stale working trees.
+reflog, rerere metadata or stale working trees. May also update ancillary
+indexes such as the commit-graph.
 
 Users are encouraged to run this task on a regular basis within
 each repository to maintain good disk space utilization and good

2.19.1.390.gf3a00b506f