bug#31706: Emacs 26.1 unable to visit files in vc-git buffer's links after vc-pull

[Van L]

Hello.

After a vc-pull on a git directory, a vc-git buffer shows with links to files 
that have changed.

— example
Running "git pull"...
From git://github.com/apple/foundationdb
   ab5bfad..ce6a2f0  master     -> origin/master
Updating ab5bfad..ce6a2f0
Fast-forward
 fdbclient/CoordinationInterface.h     |  2 +-
 fdbclient/DatabaseContext.h           |  3 +-
 fdbclient/Knobs.cpp                   |  1 +
 fdbclient/Knobs.h                     |  1 +
 fdbclient/MasterProxyInterface.h      |  2 +-
 fdbclient/MonitorLeader.actor.cpp     | 84 +++++++++++++++++++----------------
 fdbclient/NativeAPI.actor.cpp         | 17 ++++---
 fdbclient/NativeAPI.h                 |  2 +-
 fdbclient/ReadYourWrites.h            |  1 -
 fdbrpc/batcher.actor.h                | 20 ++++++---
 fdbserver/Knobs.cpp                   |  4 ++
 fdbserver/Knobs.h                     |  4 ++
 fdbserver/LeaderElection.actor.cpp    | 30 ++++++-------
 fdbserver/MasterProxyServer.actor.cpp | 32 +++++++++----
 fdbserver/fdbserver.actor.cpp         |  7 ++-
 flow/error_definitions.h              |  1 +
 flow/network.h                        |  1 +
 17 files changed, 131 insertions(+), 81 deletions(-)

— end example

Clicking on a link causes the ,system default open, to call the operating 
system’s file manager to open that file.

Expected desired behavior is that Emacs 26.1 simply visits the ,flow/network.h, 
file (on event ,mouse-1: visit this file,)

Trying to find the elisp function the mouse click calls, I do:

 1. C-h k
 2. <mouse click on ,flow/network.h,>
 3. the echo area shows: mouse-1-

if I click the mouse a second time I get the explanation for: 
(mouse-drag-region START-EVENT)

From experience in Org Mode I know C-x customize ; search for ,org app, ; you 
can set Emacs to be the default app for unrecognized files.

After C-x customize, I search for

 A. git app
 B. vc app

but there is no setting like `Org File Apps` to let Emacs open those links in 
the vc-git buffer.



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    '(then you'll like Daniel Stone's "The Food Explorer".))