enhance: respect git config merge.ff (#1495)

This adds another configuration possibility for the default merge mode
to the already existing per branch git configuration
`branch.<name>.mergeoptions` (#540) and the per repo preference
"Preferred Merge Mode" (#1156).

Defined values:
 - `false`: do no fast-forward merge
 - `only`: do only a fast-forward merge

This is configurable by for example `git config set --global merge.ff
<value>` for a global (meaning the current user) configuration.

The priority between these configurations/preferences is:

 - git configuration `branch.<name>.mergeoptions` (#540)
 - git configuration `merge.ff`
 - preference "Preferred Merge Mode" (#1156)

Author: sina-hide

src/Models/MergeMode.cs

@@ -2,13 +2,28 @@
 {
     public class MergeMode
     {
+        public static readonly MergeMode Default =
+            new MergeMode("Default", "Fast-forward if possible", "");
+
+        public static readonly MergeMode FastForward =
+            new MergeMode("Fast-forward", "Refuse to merge when fast-forward is not possible", "--ff-only");
+
+        public static readonly MergeMode NoFastForward =
+            new MergeMode("No Fast-forward", "Always create a merge commit", "--no-ff");
+
+        public static readonly MergeMode Squash =
+            new MergeMode("Squash", "Squash merge", "--squash");
+
+        public static readonly MergeMode DontCommit
+            = new MergeMode("Don't commit", "Merge without commit", "--no-ff --no-commit");
+
         public static readonly MergeMode[] Supported =
         [
-            new MergeMode("Default", "Fast-forward if possible", ""),
-            new MergeMode("Fast-forward", "Refuse to merge when fast-forward is not possible", "--ff-only"),
-            new MergeMode("No Fast-forward", "Always create a merge commit", "--no-ff"),
-            new MergeMode("Squash", "Squash merge", "--squash"),
-            new MergeMode("Don't commit", "Merge without commit", "--no-ff --no-commit"),
+            Default,
+            FastForward,
+            NoFastForward,
+            Squash,
+            DontCommit,
         ];
 
         public string Name { get; set; }

src/ViewModels/Merge.cs

@@ -34,7 +34,7 @@ public Merge(Repository repo, Models.Branch source, string into, bool forceFastF
 
             Source = source;
             Into = into;
-            Mode = forceFastForward ? Models.MergeMode.Supported[1] : AutoSelectMergeMode();
+            Mode = forceFastForward ? Models.MergeMode.FastForward : AutoSelectMergeMode();
         }
 
         public Merge(Repository repo, Models.Commit source, string into)
@@ -82,27 +82,49 @@ public override Task<bool> Sure()
         }
 
         private Models.MergeMode AutoSelectMergeMode()
+        {
+            return
+                GetGitConfigBranchMergeOptions()
+                ?? GetGitConfigMergeFF()
+                ?? GetSettingsPreferredMergeMode();
+        }
+
+        private Models.MergeMode GetSettingsPreferredMergeMode()
         {
             var preferredMergeModeIdx = _repo.Settings.PreferredMergeMode;
             if (preferredMergeModeIdx < 0 || preferredMergeModeIdx > Models.MergeMode.Supported.Length)
                 preferredMergeModeIdx = 0;
 
             var defaultMergeMode = Models.MergeMode.Supported[preferredMergeModeIdx];
-            var config = new Commands.Config(_repo.FullPath).Get($"branch.{Into}.mergeoptions");
-            if (string.IsNullOrEmpty(config))
-                return defaultMergeMode;
-            if (config.Equals("--ff-only", StringComparison.Ordinal))
-                return Models.MergeMode.Supported[1];
-            if (config.Equals("--no-ff", StringComparison.Ordinal))
-                return Models.MergeMode.Supported[2];
-            if (config.Equals("--squash", StringComparison.Ordinal))
-                return Models.MergeMode.Supported[3];
-            if (config.Equals("--no-commit", StringComparison.Ordinal) || config.Equals("--no-ff --no-commit", StringComparison.Ordinal))
-                return Models.MergeMode.Supported[4];
-
             return defaultMergeMode;
         }
 
+        private Models.MergeMode GetGitConfigMergeFF()
+        {
+            var config = new Commands.Config(_repo.FullPath).Get("merge.ff");
+            return config switch
+            {
+                null or "" => null,
+                "false" => Models.MergeMode.NoFastForward,
+                "only" => Models.MergeMode.FastForward,
+                _ => null
+            };
+        }
+
+        private Models.MergeMode GetGitConfigBranchMergeOptions()
+        {
+            var config = new Commands.Config(_repo.FullPath).Get($"branch.{Into}.mergeoptions");
+            return config switch
+            {
+                null or "" => null,
+                "--ff-only" => Models.MergeMode.FastForward,
+                "--no-ff" => Models.MergeMode.NoFastForward,
+                "--squash" => Models.MergeMode.Squash,
+                "--no-commit" or "--no-ff --no-commit" => Models.MergeMode.DontCommit,
+                _ => null
+            };
+        }
+
         private readonly Repository _repo = null;
         private readonly string _sourceName;
     }