merge: fix overlap between GIT_MERGE_FILE_FAVOR__CONFLICTED and GIT_MERGE_FILE_SIMPLIFY_ALNUM

[boretrk]

Somewhere along the line GIT_MERGE_FILE_FAVOR__CONFLICTED seems to have moved from git_merge_file_favor_t to git_merge_file_flag_t without changing value, causing a possible conflict in file_opts.flags

This PR allocates flags in the enums of git2/merge.h for internal usage but marks them as such to avoid overlapping bits in the future.
The flags are #defined to their previous descriptive names in src/merge.h to keep their enum type.

[ethomson]

Ugh! Thanks for catching this. Let's just make these public? There's no real good reason not to...

How do you feel about...

commit 0fbf62cd2027e4ad6aed1d207099c17d2414073c
Author: Edward Thomson <ethomson@edwardthomson.com>
Date:   Sat Feb 12 08:46:55 2022 -0500

    merge: make the internal flags public
    
    We made the flags that enable recursive merge internal, on the
    assumption that nobody would want them and they're hard to reason about.
    (Giving people an option that nobody wants is just extra noise.)
    
    However, it made it hard for _us_ to reason about.  There's no good
    reason to keep it private, let's just make it public and push that
    cognitive load onto our poor users.  But they should expect it, they're
    dealing with git, after all.

diff --git a/include/git2/merge.h b/include/git2/merge.h
index a30833915..e90941a49 100644
--- a/include/git2/merge.h
+++ b/include/git2/merge.h
@@ -93,8 +93,13 @@ typedef enum {
 	 */
 	GIT_MERGE_NO_RECURSIVE = (1 << 3),
 
-	/* This flag is reserved for internal library use */
-	GIT_MERGE__INTERNAL_FLAG = (1 << 30)
+	/**
+	 * Treat this merge as if it is to produce the virtual base
+	 * of a recursive merge.  This will ensure that there are
+	 * no conflicts, any conflicting regions will keep conflict
+	 * markers in the merge result.
+	 */
+	GIT_MERGE_VIRTUAL_BASE = (1 << 4)
 } git_merge_flag_t;
 
 /**
@@ -167,8 +172,12 @@ typedef enum {
 	/** Create zdiff3 ("zealous diff3")-style files */
 	GIT_MERGE_FILE_STYLE_ZDIFF3 = (1 << 8),
 
-	/* This flag is reserved for internal library use */
-	GIT_MERGE_FILE__INTERNAL_FLAG = (1 << 30)
+	/**
+	 * Do not produce file conflicts when common regions have
+	 * changed; keep the conflict markers in the file and accept
+	 * that as the merge result.
+	 */
+	GIT_MERGE_FILE_ACCEPT_CONFLICTS = (1 << 9)
 } git_merge_file_flag_t;
 
 #define GIT_MERGE_CONFLICT_MARKER_SIZE	7
diff --git a/src/merge.c b/src/merge.c
index 5f3a37bbe..641b32632 100644
--- a/src/merge.c
+++ b/src/merge.c
@@ -2116,11 +2116,11 @@ int git_merge__iterators(
 	file_opts.flags = opts.file_flags;
 
 	/* use the git-inspired labels when virtual base building */
-	if (opts.flags & GIT_MERGE__VIRTUAL_BASE) {
+	if (opts.flags & GIT_MERGE_VIRTUAL_BASE) {
 		file_opts.ancestor_label = "merged common ancestors";
 		file_opts.our_label = "Temporary merge branch 1";
 		file_opts.their_label = "Temporary merge branch 2";
-		file_opts.flags |= GIT_MERGE_FILE__CONFLICTED;
+		file_opts.flags |= GIT_MERGE_FILE_ACCEPT_CONFLICTS;
 		file_opts.marker_size = GIT_MERGE_CONFLICT_MARKER_SIZE + 2;
 	}
 
@@ -2280,7 +2280,7 @@ static int create_virtual_base(
 		memcpy(&virtual_opts, opts, sizeof(git_merge_options));
 
 	virtual_opts.flags &= ~GIT_MERGE_FAIL_ON_CONFLICT;
-	virtual_opts.flags |= GIT_MERGE__VIRTUAL_BASE;
+	virtual_opts.flags |= GIT_MERGE_VIRTUAL_BASE;
 
 	if ((merge_annotated_commits(&index, NULL, repo, one, two,
 			recursion_level + 1, &virtual_opts)) < 0)
diff --git a/src/merge.h b/src/merge.h
index e033e876a..23932905e 100644
--- a/src/merge.h
+++ b/src/merge.h
@@ -25,16 +25,6 @@
 #define GIT_MERGE_DEFAULT_RENAME_THRESHOLD	50
 #define GIT_MERGE_DEFAULT_TARGET_LIMIT		1000
 
-
-/** Internal merge flags. */
-
-/** The merge is for a virtual base in a recursive merge. */
-#define GIT_MERGE__VIRTUAL_BASE (GIT_MERGE__INTERNAL_FLAG)
-
-/** Accept the conflict file, staging it as the merge result. */
-#define GIT_MERGE_FILE__CONFLICTED (GIT_MERGE_FILE__INTERNAL_FLAG)
-
-
 /** Types of changes when files are merged from branch to branch. */
 typedef enum {
 	/* No conflict - a change only occurs in one branch. */
diff --git a/src/merge_driver.c b/src/merge_driver.c
index 66b257a4f..19b35ac0e 100644
--- a/src/merge_driver.c
+++ b/src/merge_driver.c
@@ -93,7 +93,7 @@ int git_merge_driver__builtin_apply(
 		goto done;
 
 	if (!result.automergeable &&
-		!(file_opts.flags & GIT_MERGE_FILE__CONFLICTED)) {
+		!(file_opts.flags & GIT_MERGE_FILE_ACCEPT_CONFLICTS)) {
 		error = GIT_EMERGECONFLICT;
 		goto done;
 	}

[boretrk]

Sure, I don't see any problem with making them public.
It's not like anyone is forced to set them.

[ethomson]

Thanks @boretrk !