git-svn: Correctly handle root commits in mergeinfo ranges

If the bottom of a mergeinfo range is a commit that maps to a git root
commit, then it doesn't have a parent.  In such a case, use git commit
range "$top_commit" rather than "$bottom_commit^..$top_commit".

[ew: line-wrap at 80 columns]

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Acked-by: Eric Wong <normalperson@yhbt.net>

git-svn.perl

@@ -3124,8 +3124,12 @@ sub lookup_svn_merge {
 			next;
 		}
 
-		push @merged_commit_ranges,
-			"$bottom_commit^..$top_commit";
+		if (scalar(command('rev-parse', "$bottom_commit^@"))) {
+			push @merged_commit_ranges,
+			     "$bottom_commit^..$top_commit";
+		} else {
+			push @merged_commit_ranges, "$top_commit";
+		}
 
 		if ( !defined $tip or $top > $tip ) {
 			$tip = $top;

t/t9159-git-svn-no-parent-mergeinfo.sh

@@ -2,7 +2,7 @@
 test_description='git svn handling of root commits in merge ranges'
 . ./lib-git-svn.sh
 
-test_expect_failure 'test handling of root commits in merge ranges' '
+test_expect_success 'test handling of root commits in merge ranges' '
 	mkdir -p init/trunk init/branches init/tags &&
 	echo "r1" > init/trunk/file.txt &&
 	svn_cmd import -m "initial import" init "$svnrepo" &&