Ignore missing MERGE_MSG when deleting MERGE_MSG

Repository.writeMergeCommitMsg(null) no longer fails if the MERGE_MSG
file is missing. This was done to avoid CommitCommand to fail in case of
a missing MERGE_MSG file.

Bug: 352243
Change-Id: Iddf43533d133f8f22199ed6e2393a552670e7d1f
Signed-off-by: Jens Baumgart <jens.baumgart@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/lib/Repository.java

@@ -1137,7 +1137,7 @@ public abstract class Repository {
 				fos.close();
 			}
 		} else {
-			FileUtils.delete(mergeMsgFile);
+			FileUtils.delete(mergeMsgFile, FileUtils.SKIP_MISSING);
 		}
 	}