Address nightly test failures due to git-diff(1) behaviour change

The nightly tests are failing due to behavioural changes in git-diff(1) introduced in b55e6d36eb (diff: ensure consistent diff behavior with ignore options, 2025-08-08): git@b55e6d36

Gitaly's diffBlob function executes git-diff(1) with the following significant flags:

--patch-with-raw --abbrev=40 --ignore-space-change

• --patch-with-raw allows us to produce output where the first X lines are diff-format, followed by a newline, followed by the standard patch body.
• --ignore-space-change is supposed to short-circuit the diff if the only changes are whitespace-related. This is controlled via an RPC option.

There is a breaking change in the output of git-diff(1) with --ignore-space-change present on the next branch.

2.50.1

On Git 2.50.1, invoking git-diff(1) with the same arguments as Gitaly in addition to --no-pager yields:

❯ git --no-pager --git-dir /tmp/gitaly-364529736/292607228/storages.d/default/@hashed/91/46/91468c6323986bf2c002a39f5db53871080bd84d208355d4dc7e78d5f73015c8.git -c gc.auto=0 -c maintenance.auto=0 -c core.autocrlf=false -c core.useReplaceRefs=false -c core.bigFileThreshold=50m -c p  
ack.allowPackReuse=multi -c core.fsync=objects,derived-metadata,reference -c core.fsyncMethod=fsync -c core.packedRefsTimeout=10000 -c core.filesRefLockTimeout=1000 -c attr.tree=HEAD diff --patch-with-raw --abbrev=40 --ignore-space-change --end-of-options 257cc5642cb1a054f08cc83f2  
d943e56fd3ebe99 564d4ddac63d470827491965b3299872fa67602d                                                                                                                                                                                                   
:100644 100644 257cc5642cb1a054f08cc83f2d943e56fd3ebe99 564d4ddac63d470827491965b3299872fa67602d M      257cc5642cb1a054f08cc83f2d943e56fd3ebe99

There is one "raw" line produced which is read by the parser here. The parser continues uneventfully, eventually exiting here.

next

On next, 662b1ed5c5d90a08b6a38b48f842423f2f746b3f, the command yields no output:

❯ /home/jliu/code/git/bin-wrappers/git --no-pager --git-dir /tmp/gitaly-364529736/292607228/storages.d/default/@hashed/91/46/91468c6323986bf2c002a39f5db53871080bd84d208355d4dc7e78d5f73015c8.git -c gc.auto=0 -c maintenance.auto=0 -c core.autocrlf=false -c core.useReplaceRefs=false  
-c core.bigFileThreshold=50m -c pack.allowPackReuse=multi -c core.fsync=objects,derived-metadata,reference -c core.fsyncMethod=fsync -c core.packedRefsTimeout=10000 -c core.filesRefLockTimeout=1000 -c attr.tree=HEAD diff --patch-with-raw --abbrev=40 --ignore-space-change --end-of-  
options 257cc5642cb1a054f08cc83f2d943e56fd3ebe99 564d4ddac63d470827491965b3299872fa67602d

When it comes time to Parse(), Gitaly finds that no "raw" lines were cached, and the parser exits prematurely with an error.

---

It seems like we're currently relying on the fact that git-diff(1) with the --ignore-space-change option will produce a diff-format line followed by an empty patch. However, in git@b55e6d36, this behaviour was changed so that no diff-format lines will be produced at all.