Added -[GTCommit merge:error:] to quickly merge two commits together

(Essentially a wrapper around git_merge_commits())

Author: slavikus

ObjectiveGit/GTCommit.h

@@ -33,6 +33,7 @@
 @class GTSignature;
 @class GTTree;
 @class GTOID;
+@class GTIndex;
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -54,6 +55,17 @@ NS_ASSUME_NONNULL_BEGIN
 /// The underlying `git_object` as a `git_commit` object.
 - (git_commit *)git_commit __attribute__((objc_returns_inner_pointer));
 
+/// Merges the given commit into the receiver in memory and produces the result as
+/// an index.
+///
+/// otherTree    - The commit with which the receiver should be merged with. Cannot be
+///                nil.
+/// error        - The error if one occurred.
+///
+/// Returns an index which represents the result of the merge, or nil if an error
+/// occurred.
+- (nullable GTIndex *)merge:(GTCommit *)otherCommit error:(NSError **)error;
+
 @end
 
 NS_ASSUME_NONNULL_END

ObjectiveGit/GTCommit.m

@@ -35,9 +35,11 @@
 #import "NSString+Git.h"
 #import "NSDate+GTTimeAdditions.h"
 #import "GTOID.h"
+#import "GTIndex.h"
 
 #import "git2/commit.h"
 #import "git2/errors.h"
+#import "git2/merge.h"
 
 @implementation GTCommit
 
@@ -130,4 +132,20 @@ - (NSArray *)parents {
 	return parents;
 }
 
+#pragma mark Merging
+
+- (GTIndex *)merge:(GTCommit *)otherCommit error:(NSError **)error {
+	NSParameterAssert(otherCommit != nil);
+	
+	git_index *index;
+	
+	int result = git_merge_commits(&index, self.repository.git_repository, self.git_commit, otherCommit.git_commit, NULL);
+	if (result != GIT_OK || index == NULL) {
+		if (error != NULL) *error = [NSError git_errorFor:result description:@"Failed to merge commit %@ with commit %@", self.SHA, otherCommit.SHA];
+		return nil;
+	}
+	
+	return [[GTIndex alloc] initWithGitIndex:index repository:self.repository];
+}
+
 @end