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[PATCH 3/3] commit-reach.h: add missing declarations (hdr-check)

Add the necessary #includes and forward declarations to allow the header
file to pass the 'hdr-check' target.

Note that, since this header includes the commit-slab implementation
header file (indirectly via commit-slab.h), some of the commit-slab
inline functions (e.g contains_cache_at_peek()) will not compile without
the complete type of 'struct commit'. Hence, we replace the forward
declaration of 'struct commit' with the an #include of the 'commit.h'
header file.

It is possible, using the 'commit-slab-{decl,impl}.h' files, to avoid
this inclusion of the 'commit.h' header. Commit a9f1f1f9f8 ("commit-slab.h:
code split", 2018-05-19) separated the commit-slab interface from its
implementation, to allow for the definition of a public commit-slab data
structure. This enabled us to avoid including the commit-slab implementation
in a header file, which could result in the replication of the commit-slab
functions in each compilation unit in which it was included.

Indeed, if you compile with optimizations disabled, then run this script:

  $ cat -n dup-static.sh
       1 #!/bin/sh
       3 nm $1 | grep ' t ' | cut -d' ' -f3 | sort | uniq -c |
       4 	sort -rn | grep -v '      1'

  $ ./dup-static.sh git | grep contains
       24 init_contains_cache_with_stride
       24 init_contains_cache
       24 contains_cache_peek
       24 contains_cache_at_peek
       24 contains_cache_at
       24 clear_contains_cache

you will find 24 copies of the commit-slab routines for the contains_cache.
Of course, when you enable optimizations again, these duplicate static
functions (mostly) disappear. Compiling with gcc at -O2, leaves two static
functions, thus:

  $ nm commit-reach.o | grep contains_cache
  0000000000000870 t contains_cache_at_peek.isra.1.constprop.6
  $ nm ref-filter.o | grep contains_cache
  00000000000002b0 t clear_contains_cache.isra.14

However, using a shared 'contains_cache' would result in all six of the
above functions as external public functions in the git binary. At present,
only three of these functions are actually called, so the trade-off
seems to favour letting the compiler inline the commit-slab functions.

Signed-off-by: Ramsay Jones <ramsay@xxxxxxxxxxxxxxxxxxxx>
 commit-reach.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/commit-reach.h b/commit-reach.h
index 7d313e2975..f41d8f6ba3 100644
--- a/commit-reach.h
+++ b/commit-reach.h
@@ -1,12 +1,13 @@
 #ifndef __COMMIT_REACH_H__
 #define __COMMIT_REACH_H__
+#include "commit.h"
 #include "commit-slab.h"
-struct commit;
 struct commit_list;
-struct contains_cache;
 struct ref_filter;
+struct object_id;
+struct object_array;
 struct commit_list *get_merge_bases_many(struct commit *one,
 					 int n,