Lib-ify fmt-merge-msg

In builtin.h, there exists the distinctly "lib-ish" function
fmt_merge_msg(). This function can currently only be called by built-in
commands but, unlike most of the other functions in the header, it does
not make sense to impose this restriction as the functionality can be
logically reused in libgit.

Extract this function into fmt-merge-msg.c so that related definitions
can exist clearly in their own header file.

While we're at it, clean up #includes that are unused.

This patch is best viewed with --color-moved.

Signed-off-by: Denton Liu <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Denton-L

Makefile

@@ -886,6 +886,7 @@ LIB_OBJS += ewah/ewah_rlw.o
 LIB_OBJS += exec-cmd.o
 LIB_OBJS += fetch-negotiator.o
 LIB_OBJS += fetch-pack.o
+LIB_OBJS += fmt-merge-msg.o
 LIB_OBJS += fsck.o
 LIB_OBJS += fsmonitor.o
 LIB_OBJS += gettext.o

builtin.h

@@ -94,8 +94,6 @@
  * command.
  */
 
-#define DEFAULT_MERGE_LOG_LEN 20
-
 extern const char git_usage_string[];
 extern const char git_more_info_string[];
 
@@ -104,15 +102,6 @@ extern const char git_more_info_string[];
 
 void prune_packed_objects(int);
 
-struct fmt_merge_msg_opts {
-	unsigned add_title:1,
-		credit_people:1;
-	int shortlog_len;
-};
-
-int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
-		  struct fmt_merge_msg_opts *);
-
 /**
  * If a built-in has DELAY_PAGER_CONFIG set, the built-in should call this early
  * when it wishes to respect the `pager.foo`-config. The `cmd` is the name of