When splitting a subtree, committs that do not intersect prefix
receive identity mapping (oldrev -> oldrev). If such commit
is reachable by multiple paths in the revision DAG, the cache_set()
function may be called twice for the same (oldrev -> newrev) pair.

This triggers fatal error "cache for <hash> already exists"

Bugfix is to make cache_set() idempotent when the same
(oldrev -> newrev) pair appears multiple times.

Signed-off-by: Roland Conybeare <redacted>
---
 contrib/subtree/git-subtree.sh | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/contrib/subtree/git-subtree.sh b/contrib/subtree/git-subtree.sh
index 791fd8260c..64590e05e0 100755
--- a/contrib/subtree/git-subtree.sh
+++ b/contrib/subtree/git-subtree.sh

@@ -343,7 +343,13 @@ cache_set () {
 		test "$oldrev" != "latest_new" &&
 		test -e "$cachedir/$oldrev"
 	then
-		die "fatal: cache for $oldrev already exists!"
+		existing=$(cat "$cachedir/$oldrev")
+		if test "$existing" = "$newrev"
+		then
+			return
+		else
+			die "fatal: cache for $oldrev already exists!"
+		fi
 	fi
 	echo "$newrev" >"$cachedir/$oldrev"
 }

-- 
2.50.1