Honor core.precomposeUnicode in more places

On Mac's HFS where git sets core.precomposeUnicode to true automatically
by git init/clone, when a user creates a simple unicode refname (in NFC
format) such as españa:

  $ git branch españa

different commands would display the branch name differently.  For
example, git branch, git log --decorate, and git fast-export all used

  65 73 70 61 c3 b1 61  (or "espa\xc3\xb1a")

(NFC form) while show-ref would use

  65 73 70 61 6e cc 83 61  (or "espan\xcc\x83a")

(NFD form).  A stress test for git filter-repo was tripped up by this
inconsistency, though digging in I found that the problems could
compound; for example, if the user ran

  $ git pack-refs --all

and then tried to check out the branch, they would be met with:

  $ git checkout españa
  error: pathspec 'españa' did not match any file(s) known to git

  $ git checkout españa --
  fatal: invalid reference: españa

  $ git branch
    españa
  * master

Note that the user could run the `git branch` command first and copy and
paste the `españa` portion of the output and still see the same two
errors.  Also, if the user added --no-prune to the pack-refs command,
then they would see three branches: master, españa, and españa (those
last two are NFC vs. NFD forms, even if they render the same).

Further, if the user had the `españa` branch checked out before
running `git pack-refs --all`, the user would be greeted with (note
that I'm trimming trailing output with an ellipsis):

  $ git rev-parse HEAD
  fatal: ambiguous argument 'HEAD': unknown revision or path...

  $ git status
  On branch españa

  No commits yet...

Or worse, if the user didn't check this stuff first, running `git
commit` will create a new commit with all changes of all of history
being squashed into it.

In addition to pack-refs, one could also get into this state with
upload-pack or anything that calls either pack-refs or upload-pack (e.g.
gc or clone).

Add code in a few places (pack-refs, show-ref, upload-pack) to check and
honor the setting of core.precomposeUnicode to avoid these bugs.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-refs.c

@@ -1,4 +1,5 @@
 #include "builtin.h"
+#include "config.h"
 #include "parse-options.h"
 #include "refs.h"
 #include "repository.h"
@@ -16,6 +17,7 @@ int cmd_pack_refs(int argc, const char **argv, const char *prefix)
 		OPT_BIT(0, "prune", &flags, N_("prune loose refs (default)"), PACK_REFS_PRUNE),
 		OPT_END(),
 	};
+	git_config(git_default_config, NULL);
 	if (parse_options(argc, argv, prefix, opts, pack_refs_usage, 0))
 		usage_with_options(pack_refs_usage, opts);
 	return refs_pack_refs(get_main_ref_store(the_repository), flags);

builtin/show-ref.c

@@ -1,5 +1,6 @@
 #include "builtin.h"
 #include "cache.h"
+#include "config.h"
 #include "refs.h"
 #include "object-store.h"
 #include "object.h"
@@ -182,6 +183,8 @@ static const struct option show_ref_options[] = {
 
 int cmd_show_ref(int argc, const char **argv, const char *prefix)
 {
+	git_config(git_default_config, NULL);
+
 	argc = parse_options(argc, argv, prefix, show_ref_options,
 			     show_ref_usage, 0);
 

upload-pack.c

@@ -1041,6 +1041,8 @@ static int upload_pack_config(const char *var, const char *value, void *unused)
 		allow_filter = git_config_bool(var, value);
 	} else if (!strcmp("uploadpack.allowrefinwant", var)) {
 		allow_ref_in_want = git_config_bool(var, value);
+	} else if (!strcmp("core.precomposeunicode", var)) {
+		precomposed_unicode = git_config_bool(var, value);
 	}
 
 	if (current_config_scope() != CONFIG_SCOPE_REPO) {