shallow: don't default to -1 for depth

Depth of `0` should indicate full depth. Disallow negative values (they
may have a future meaning) and use `0` as the default.

include/git2/remote.h

@@ -744,9 +744,9 @@ typedef struct {
 	git_proxy_options proxy_opts;
 
 	/**
-	 * Depth of the fetch to perform. Depth <= 0 fetches the full history.
+	 * Depth of the fetch to perform, or 0 for full history.
 	 *
-	 * The default is -1.
+	 * The default is 0.
 	 */
 	int depth;
 
@@ -772,7 +772,7 @@ typedef struct {
 
 #define GIT_FETCH_OPTIONS_VERSION 1
 #define GIT_FETCH_OPTIONS_INIT { GIT_FETCH_OPTIONS_VERSION, GIT_REMOTE_CALLBACKS_INIT, GIT_FETCH_PRUNE_UNSPECIFIED, 1, \
-				 GIT_REMOTE_DOWNLOAD_TAGS_UNSPECIFIED, GIT_PROXY_OPTIONS_INIT, -1, 0 }
+				 GIT_REMOTE_DOWNLOAD_TAGS_UNSPECIFIED, GIT_PROXY_OPTIONS_INIT }
 
 /**
  * Initialize git_fetch_options structure

src/libgit2/clone.c

@@ -421,7 +421,7 @@ static int clone_into(
 	memcpy(&fetch_opts, opts, sizeof(git_fetch_options));
 	fetch_opts.update_fetchhead = 0;
 
-	if (opts->depth <= 0)
+	if (!opts->depth)
 		fetch_opts.download_tags = GIT_REMOTE_DOWNLOAD_TAGS_ALL;
 
 	if ((error = git_remote_connect_options__from_fetch_opts(&connect_opts, remote, &fetch_opts)) < 0)

src/libgit2/fetch.c

@@ -172,10 +172,12 @@ int git_fetch_negotiate(git_remote *remote, const git_fetch_options *opts)
 
 	remote->need_pack = 0;
 
-	if (!opts)
-		remote->nego.depth = -1;
-	else
+	if (opts) {
+		GIT_ASSERT_ARG(opts->unshallow == 0 || opts->depth == 0);
+		GIT_ASSERT_ARG(opts->depth >= 0);
+
 		remote->nego.depth = opts->unshallow ? INT_MAX : opts->depth;
+	}
 
 	if (filter_wants(remote, opts) < 0)
 		return -1;
@@ -184,11 +186,6 @@ int git_fetch_negotiate(git_remote *remote, const git_fetch_options *opts)
 	if (!remote->need_pack)
 		return 0;
 
-	if (opts && opts->unshallow && opts->depth > 0) {
-		git_error_set(GIT_ERROR_INVALID, "options '--depth' and '--unshallow' cannot be used together");
-		return -1;
-	}
-
 	/*
 	 * Now we have everything set up so we can start tell the
 	 * server what we want and what we have.