fetchpatch: add `relative`

Allows restricting patches to a specific subdirectory, à la
`git diff --relative=subdir`.

This cannot be done (cleanly) currently because the `includes` logic
happens *after* `stripLen` is applied, so we can't match on `subdir/*`.

This change adds a `relative` argument that makes this possible by
filtering files before doing any processing, and setting `stripLen` and
`extraPrefix` accordingly.

1 files changed, 3 insertions, 2 deletions

diff --git a/doc/contributing/coding-conventions.chapter.md b/doc/contributing/coding-conventions.chapter.md
index cfe8582e514a4..dac6d828ac0b7 100644
--- a/doc/contributing/coding-conventions.chapter.md
+++ b/doc/contributing/coding-conventions.chapter.md
@@ -540,10 +540,11 @@ If you do need to do create this sort of patch file, one way to do so is with gi
 
 If a patch is available online but does not cleanly apply, it can be modified in some fixed ways by using additional optional arguments for `fetchpatch`:
 
+- `relative`: Similar to using `git-diff`'s `--relative` flag, only keep changes inside the specified directory, making paths relative to it.
 - `stripLen`: Remove the first `stripLen` components of pathnames in the patch.
 - `extraPrefix`: Prefix pathnames by this string.
-- `excludes`: Exclude files matching this pattern.
-- `includes`: Include only files matching this pattern.
+- `excludes`: Exclude files matching these patterns (applies after the above arguments).
+- `includes`: Include only files matching these patterns (applies after the above arguments).
 - `revert`: Revert the patch.
 
 Note that because the checksum is computed after applying these effects, using or modifying these arguments will have no effect unless the `sha256` argument is changed as well.