index: preserve extension parsing errors

Previously, we would clobber any extension-specific error message with
an "extension is truncated" message. This makes `read_extension`
correctly preserve those errors, takes responsibility for truncation
errors, and adds a new message with the actual extension signature for
unsupported mandatory extensions.

1 files changed, 3 insertions, 0 deletions

diff --git a/docs/changelog.md b/docs/changelog.md
index 26899a585..bf31ec9d8 100644
--- a/docs/changelog.md
+++ b/docs/changelog.md
@@ -71,6 +71,9 @@ v0.27 + 1
 * Revision walks are now more efficient when the output is unsorted;
   we now avoid walking all the way to the beginning of history unnecessarily.
 
+* Error-handling around index extension loading has been fixed. We were
+  previously always misreporting a truncated index (#4858).
+
 ### API additions
 
 * The index may now be iterated atomically using `git_index_iterator`.