http.c: inline set_curl_keepalive()

At the end of `get_curl_handle()` we call `set_curl_keepalive()` to
enable TCP keepalive probes on our CURL handle. `set_curl_keepalive()`
dates back to 47ce115370 (http: use curl's tcp keepalive if available,
2013-10-14), which conditionally compiled different variants of
`set_curl_keepalive()` depending on what version of curl we were
compiled with[^1].

As of f7c094060c (git-curl-compat: remove check for curl 7.25.0,
2024-10-23), we no longer conditionally compile `set_curl_keepalive()`
since we no longer support pre-7.25.0 versions of curl. But the version
of that function that we kept is really just a thin wrapper around
setting the TCP_KEEPALIVE option, so there's no reason to keep it in its
own function.

Inline the definition of `set_curl_keepalive()` to within
`get_curl_handle()` so that the setup of our CURL handle is
self-contained.

[1]: The details are spelled out in 47ce115370, but the gist is curl
  7.25.0 and newer use CURLOPT_TCP_KEEPALIVE, older versions use
  CURLOPT_SOCKOPTFUNCTION with a custom callback, and older versions
  that predate even that option do nothing.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.c

@@ -704,10 +704,6 @@ static int has_proxy_cert_password(void)
 	return 1;
 }
 
-static void set_curl_keepalive(CURL *c)
-{
-	curl_easy_setopt(c, CURLOPT_TCP_KEEPALIVE, 1);
-}
 
 /* Return 1 if redactions have been made, 0 otherwise. */
 static int redact_sensitive_header(struct strbuf *header, size_t offset)
@@ -1242,7 +1238,7 @@ static CURL *get_curl_handle(void)
 	}
 	init_curl_proxy_auth(result);
 
-	set_curl_keepalive(result);
+	curl_easy_setopt(result, CURLOPT_TCP_KEEPALIVE, 1);
 
 	return result;
 }