Bump pub to 867ad175d81713c4e70473b1ab32107547cbdc3e

Changes:
```
> git log --format="%C(auto) %h %s" 6fdcdd4..867ad17
 https://dart.googlesource.com/pub.git/+/867ad175 Fix reading pubspecs from git (#3670)

```

Diff: https://dart.googlesource.com/pub.git/+/6fdcdd4357645817e7d7027ee2157ed68ef69c53~..867ad175d81713c4e70473b1ab32107547cbdc3e/
Bug: flutter/flutter#116107
Change-Id: I6b1a2f1e4f1864c9a2658568f6dfd63383d10406
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/272620
Commit-Queue: Siva Annamalai <[email protected]>
Reviewed-by: Martin Kustermann <[email protected]>
Reviewed-by: Jonas Jensen <[email protected]>
Commit-Queue: Sigurd Meldgaard <[email protected]>

Author: sigurdm

DEPS

@@ -152,7 +152,7 @@ vars = {
   "ply_rev": "604b32590ffad5cbb82e4afef1d305512d06ae93",
   "pool_rev": "1ea5b031cfda37786d305292cb8104dffb45d9ae",
   "protobuf_rev": "c181573c330ff0337db0bab5501deda3a42ba028",
-  "pub_rev": "6fdcdd4357645817e7d7027ee2157ed68ef69c53", # manually rev'd
+  "pub_rev": "867ad175d81713c4e70473b1ab32107547cbdc3e", # manually rev'd
   "pub_semver_rev": "17231113f98c80285881cc4452863bcfd085d1a8",
   "root_certificates_rev": "692f6d6488af68e0121317a9c2c9eb393eb0ee50",
   "shelf_rev": "1c2104737973715426035c11ba840c7f23d8f186",