refs: add GIT_TRACE_REFS debugging mechanism

When set in the environment, GIT_TRACE_REFS makes git print operations and
results as they flow through the ref storage backend. This helps debug
discrepancies between different ref backends.

Example:

    $ GIT_TRACE_REFS="1" ./git branch
    15:42:09.769631 refs/debug.c:26         ref_store for .git
    15:42:09.769681 refs/debug.c:249        read_raw_ref: HEAD: 0000000000000000000000000000000000000000 (=> refs/heads/ref-debug) type 1: 0
    15:42:09.769695 refs/debug.c:249        read_raw_ref: refs/heads/ref-debug: 3a238e5 (=> refs/heads/ref-debug) type 0: 0
    15:42:09.770282 refs/debug.c:233        ref_iterator_begin: refs/heads/ (0x1)
    15:42:09.770290 refs/debug.c:189        iterator_advance: refs/heads/b4 (0)
    15:42:09.770295 refs/debug.c:189        iterator_advance: refs/heads/branch3 (0)

Signed-off-by: Han-Wen Nienhuys <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: hanwen

Documentation/git.txt

@@ -722,6 +722,10 @@ of clones and fetches.
 	time of each Git command.
 	See `GIT_TRACE` for available trace output options.
 
+`GIT_TRACE_REFS`::
+	Enables trace messages for operations on the ref database.
+	See `GIT_TRACE` for available trace output options.
+
 `GIT_TRACE_SETUP`::
 	Enables trace messages printing the .git, working tree and current
 	working directory after Git has completed its setup phase.

Makefile

@@ -950,6 +950,7 @@ LIB_OBJS += rebase.o
 LIB_OBJS += ref-filter.o
 LIB_OBJS += reflog-walk.o
 LIB_OBJS += refs.o
+LIB_OBJS += refs/debug.o
 LIB_OBJS += refs/files-backend.o
 LIB_OBJS += refs/iterator.o
 LIB_OBJS += refs/packed-backend.o

refs.c

@@ -1774,6 +1774,7 @@ struct ref_store *get_main_ref_store(struct repository *r)
 		BUG("attempting to get main_ref_store outside of repository");
 
 	r->refs_private = ref_store_init(r->gitdir, REF_STORE_ALL_CAPS);
+	r->refs_private = maybe_debug_wrap_ref_store(r->gitdir, r->refs_private);
 	return r->refs_private;
 }