Make "git cat-file" exception messages include repository monogram/slug

Summary:
When throwing an exception related to output provided by git cat-file, include the repository monogram to allow potentially debugging in Git.

Closes T15661

Test Plan:
Unclear. Basically: "have a broken Git repository in Diffusion".
(However this patch changes a message only shown in case of an exception, so in the worst case we'd break the exception via an exception.)

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15661

Differential Revision: https://we.phorge.it/D25460