sub-process: move sub-process functions into separate files

Move the sub-proces functions into sub-process.h/c.  Add documentation
for the new module in Documentation/technical/api-sub-process.txt

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sub-process.h

@@ -0,0 +1,49 @@
+#ifndef SUBPROCESS_H
+#define SUBPROCESS_H
+
+#include "git-compat-util.h"
+#include "hashmap.h"
+#include "run-command.h"
+
+/*
+ * Generic implementation of background process infrastructure.
+ * See Documentation/technical/api-background-process.txt.
+ */
+
+ /* data structures */
+
+struct subprocess_entry {
+	struct hashmap_entry ent; /* must be the first member! */
+	const char *cmd;
+	struct child_process process;
+};
+
+/* subprocess functions */
+
+int cmd2process_cmp(const struct subprocess_entry *e1,
+	const struct subprocess_entry *e2, const void *unused);
+
+typedef int(*subprocess_start_fn)(struct subprocess_entry *entry);
+int subprocess_start(struct hashmap *hashmap, struct subprocess_entry *entry, const char *cmd,
+		subprocess_start_fn startfn);
+
+void subprocess_stop(struct hashmap *hashmap, struct subprocess_entry *entry);
+
+struct subprocess_entry *subprocess_find_entry(struct hashmap *hashmap, const char *cmd);
+
+/* subprocess helper functions */
+
+static inline struct child_process *subprocess_get_child_process(
+		struct subprocess_entry *entry)
+{
+	return &entry->process;
+}
+
+/*
+ * Helper function that will read packets looking for "status=<foo>"
+ * key/value pairs and return the value from the last "status" packet
+ */
+
+void subprocess_read_status(int fd, struct strbuf *status);
+
+#endif