project: Add REPO_PROJECT_FETCH_URL environment variable

Add REPO_PROJECT_FETCH_URL to Project.GetEnvVars(), which resolves to
the remote fetch URL of the project. This is useful for exposing the
URL to custom fetch commands or other external scripts.

Bug: 513329573
Change-Id: Ic2b0a83493934d16bb1152366ee4e1a2c35ea2dc
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/596121
Tested-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

project.py

@@ -680,6 +680,7 @@ class Project:
         setenv("REPO_INNERPATH", self.relpath)
         setenv("REPO_PATH", self.RelPath(local=local))
         setenv("REPO_REMOTE", self.remote.name)
+        setenv("REPO_PROJECT_FETCH_URL", self.remote.url)
 
         try:
             lrev = "" if self.manifest.IsMirror else self.GetRevisionId()