receive-pack.c: consolidate find header logic

There are two functions that have very similar logic of finding a header
value. find_commit_header, and find_header. We can conslidate the logic
by introducing a new function find_header_mem, which is equivalent to
find_commit_header except it takes a len parameter that determines how
many bytes will be read. find_commit_header and find_header can then both
call find_header_mem.

This reduces duplicate logic, as the logic for finding header values
can now all live in one place.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.h

@@ -290,12 +290,17 @@ void free_commit_extra_headers(struct commit_extra_header *extra);
 
 /*
  * Search the commit object contents given by "msg" for the header "key".
+ * Reads up to "len" bytes of "msg".
  * Returns a pointer to the start of the header contents, or NULL. The length
  * of the header, up to the first newline, is returned via out_len.
  *
  * Note that some headers (like mergetag) may be multi-line. It is the caller's
  * responsibility to parse further in this case!
  */
+const char *find_header_mem(const char *msg, size_t len,
+			const char *key,
+			size_t *out_len);
+
 const char *find_commit_header(const char *msg, const char *key,
 			       size_t *out_len);