Clarify GIT_DIFF_INCLUDE_UNTRACKED_CONTENT option

This improves the docs for GIT_DIFF_INCLUDE_UNTRACKED_CONTENT as
well as the other flags related to UNTRACKED items in diff, plus
it makes that flag now automatically turn on
GIT_DIFF_INCLUDE_UNTRACKED which seems like a reasonable dwim type
of change.

include/git2/diff.h

@@ -89,9 +89,10 @@ typedef enum {
 	/** Include unmodified files in the diff list */
 	GIT_DIFF_INCLUDE_UNMODIFIED = (1 << 9),
 
-	/** Even with GIT_DIFF_INCLUDE_UNTRACKED, an entire untracked directory
-	 *  will be marked with only a single entry in the diff list; this flag
-	 *  adds all files under the directory as UNTRACKED entries, too.
+	/** Even with GIT_DIFF_INCLUDE_UNTRACKED, an entire untracked
+	 *  directory will be marked with only a single entry in the diff list
+	 *  (a la what core Git does in `git status`); this flag adds *all*
+	 *  files under untracked directories as UNTRACKED entries, too.
 	 */
 	GIT_DIFF_RECURSE_UNTRACKED_DIRS = (1 << 10),
 
@@ -103,7 +104,11 @@ typedef enum {
 	/** Use case insensitive filename comparisons */
 	GIT_DIFF_DELTAS_ARE_ICASE = (1 << 12),
 
-	/** When generating patch text, include the content of untracked files */
+	/** When generating patch text, include the content of untracked
+	 *  files.  This automatically turns on GIT_DIFF_INCLUDE_UNTRACKED but
+	 *  it does not turn on GIT_DIFF_RECURSE_UNTRACKED_DIRS.  Add that
+	 *  flag if you want the content of every single UNTRACKED file.
+	 */
 	GIT_DIFF_INCLUDE_UNTRACKED_CONTENT = (1 << 13),
 
 	/** Disable updating of the `binary` flag in delta records.  This is
@@ -139,7 +144,7 @@ typedef enum {
 	 *  consider UNTRACKED only if it has an actual untracked file in it.
 	 *  This scan is extra work for a case you often don't care about.  This
 	 *  flag makes libgit2 immediately label an untracked directory as
-	 *  UNTRACKED without looking insde it (which differs from core Git).
+	 *  UNTRACKED without looking inside it (which differs from core Git).
 	 *  Of course, ignore rules are still checked for the directory itself.
 	 */
 	GIT_DIFF_FAST_UNTRACKED_DIRS = (1 << 19),

src/diff.c

@@ -383,6 +383,10 @@ static int diff_list_apply_options(
 	if (DIFF_FLAG_IS_SET(diff, GIT_DIFF_INCLUDE_TYPECHANGE_TREES))
 		diff->opts.flags |= GIT_DIFF_INCLUDE_TYPECHANGE;
 
+	/* flag INCLUDE_UNTRACKED_CONTENT implies INCLUDE_UNTRACKED */
+	if (DIFF_FLAG_IS_SET(diff, GIT_DIFF_INCLUDE_UNTRACKED_CONTENT))
+		diff->opts.flags |= GIT_DIFF_INCLUDE_UNTRACKED;
+
 	/* load config values that affect diff behavior */
 	if (git_repository_config__weakptr(&cfg, repo) < 0)
 		return -1;