mailinfo: libify

Move the bulk of the code from builtin/mailinfo.c to mailinfo.c
so that new callers can start calling mailinfo() directly.

Note that a few calls to exit() and die() need to be cleaned up
for the API to be truly useful, which will come in later steps.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -726,6 +726,7 @@ LIB_OBJS += list-objects.o
 LIB_OBJS += ll-merge.o
 LIB_OBJS += lockfile.o
 LIB_OBJS += log-tree.o
+LIB_OBJS += mailinfo.o
 LIB_OBJS += mailmap.o
 LIB_OBJS += match-trees.o
 LIB_OBJS += merge.o

mailinfo.h

@@ -0,0 +1,40 @@
+#ifndef MAILINFO_H
+#define MAILINFO_H
+
+#define MAX_BOUNDARIES 5
+
+struct mailinfo {
+	FILE *input;
+	FILE *output;
+	FILE *patchfile;
+
+	struct strbuf name;
+	struct strbuf email;
+	int keep_subject;
+	int keep_non_patch_brackets_in_subject;
+	int add_message_id;
+	int use_scissors;
+	int use_inbody_headers;
+	const char *metainfo_charset;
+
+	struct strbuf *content[MAX_BOUNDARIES];
+	struct strbuf **content_top;
+	struct strbuf charset;
+	char *message_id;
+	enum  {
+		TE_DONTCARE, TE_QP, TE_BASE64
+	} transfer_encoding;
+	int patch_lines;
+	int filter_stage; /* still reading log or are we copying patch? */
+	int header_stage; /* still checking in-body headers? */
+	struct strbuf **p_hdr_data;
+	struct strbuf **s_hdr_data;
+
+	struct strbuf log_message;
+};
+
+extern void setup_mailinfo(struct mailinfo *);
+extern int mailinfo(struct mailinfo *, const char *msg, const char *patch);
+extern void clear_mailinfo(struct mailinfo *);
+
+#endif /* MAILINFO_H */