Merge pull request microsoft#455: fetch/pull: use the sparse index

When I was playing around with trace2 data and creating flamegraphs, I tried a `git fetch` call to see how the `git-remote-https` command would show up. What I didn't expect was an `ensure_full_index()` region!

It turns out that `git fetch` and `git pull` need to check the index for a `.gitmodules` file to see if it should recurse into any submodules. Here is the stack trace from a debugger:

```
#0  ensure_full_index (istate=0x555555ac1c80 <the_index>) at sparse-index.c:404
microsoft#1  0x000055555571a979 in do_read_index (istate=istate@entry=0x555555ac1c80 <the_index>, path=path@entry=0x555555ad7b90 ".git/index", must_exist=must_exist@entry=0) at read-cache.c:2386
microsoft#2  0x000055555571eb7d in do_read_index (must_exist=0, path=0x555555ad7b90 ".git/index", istate=0x555555ac1c80 <the_index>) at hash.h:244
microsoft#3  read_index_from (istate=0x555555ac1c80 <the_index>, path=0x555555ad7b90 ".git/index", gitdir=0x555555ad7b30 ".git") at read-cache.c:2426
microsoft#4  0x000055555573f4c2 in repo_read_index (repo=repo@entry=0x555555ac1da0 <the_repo>) at repository.c:286
microsoft#5  0x00005555556f14d0 in get_oid_with_context_1 (repo=repo@entry=0x555555ac1da0 <the_repo>, name=name@entry=0x55555582c022 ":.gitmodules", flags=flags@entry=0, prefix=prefix@entry=0x0, 
    oid=oid@entry=0x7fffffffdb00, oc=oc@entry=0x7fffffffda70) at object-name.c:1850
microsoft#6  0x00005555556f1f53 in get_oid_with_context (oc=0x7fffffffda70, oid=0x7fffffffdb00, flags=0, str=0x55555582c022 ":.gitmodules", repo=0x555555ac1da0 <the_repo>) at object-name.c:1947
microsoft#7  repo_get_oid (r=r@entry=0x555555ac1da0 <the_repo>, name=name@entry=0x55555582c022 ":.gitmodules", oid=oid@entry=0x7fffffffdb00) at object-name.c:1603
microsoft#8  0x000055555577330f in config_from_gitmodules (fn=fn@entry=0x555555773460 <gitmodules_fetch_config>, repo=0x555555ac1da0 <the_repo>, data=data@entry=0x7fffffffdb60) at submodule-config.c:650
microsoft#9  0x000055555577462d in config_from_gitmodules (data=0x7fffffffdb60, repo=<optimized out>, fn=0x555555773460 <gitmodules_fetch_config>) at submodule-config.c:638
microsoft#10 fetch_config_from_gitmodules (max_children=<optimized out>, recurse_submodules=<optimized out>) at submodule-config.c:800
microsoft#11 0x00005555555b9e41 in cmd_fetch (argc=1, argv=0x7fffffffe090, prefix=0x0) at builtin/fetch.c:1999
microsoft#12 0x0000555555573ff6 in run_builtin (argv=<optimized out>, argc=<optimized out>, p=<optimized out>) at git.c:528
microsoft#13 handle_builtin (argc=<optimized out>, argv=<optimized out>) at git.c:785
microsoft#14 0x000055555557528c in run_argv (argv=0x7fffffffddf0, argcp=0x7fffffffddfc) at git.c:857
microsoft#15 cmd_main (argc=<optimized out>, argv=<optimized out>) at git.c:993
microsoft#16 0x0000555555573ac8 in main (argc=3, argv=0x7fffffffe088) at common-main.c:52
```

The operations these commands use are guarded by items such as `index_name_pos()` and others. Since the `.gitmodules` file is always at root, we would not need to expand, anyway.

builtin/fetch.c

@@ -2002,6 +2002,8 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 	}
 
 	git_config(git_fetch_config, NULL);
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
 
 	argc = parse_options(argc, argv, prefix,
 			     builtin_fetch_options, builtin_fetch_usage, 0);

builtin/pull.c

@@ -994,6 +994,8 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 		set_reflog_message(argc, argv);
 
 	git_config(git_pull_config, NULL);
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
 
 	argc = parse_options(argc, argv, prefix, pull_options, pull_usage, 0);