Propagate context and ensure git commands run in request context (#17868)

This PR continues the work in #17125 by progressively ensuring that git
commands run within the request context.

This now means that the if there is a git repo already open in the context it will be used instead of reopening it.

Signed-off-by: Andrew Thornton <art27@cantab.net>

models/db/engine.go

@@ -64,6 +64,7 @@ type Engine interface {
 	Distinct(...string) *xorm.Session
 	Query(...interface{}) ([]map[string][]byte, error)
 	Cols(...string) *xorm.Session
+	Context(ctx context.Context) *xorm.Session
 }
 
 // TableInfo returns table's information via an object