Turn off git diff by default in 'hg show'

Summary:
Bryan O'Sullivan found that the copy detection in git diff was a significant contributor to the relative slowness of 'hg log -vpr .' compared to 'git show'. Automatically disable git-style diff by default for 'hg show', and allow you to re-enable it via command line option if you need it and can accept the slowdown.

Depends on D2820421

Test Plan:
Run the automated test and confirm no regression.
```
: /data/users/simonfar/fb-hgext (hg) [nocopydetectioninshow]
: simonfar@devvm148 tests $ ../../hg/tests/run-tests.py -l test-show.t -i
.
# Ran 1 tests, 0 skipped, 0 warned, 0 failed.
```
Confirm that git diffs are the default in your hgrc:
```
: /data/users/simonfar/fb-hgext (hg) [nocopydetectioninshow]
: simonfar@devvm148 tests $ hg --config alias.show=show --config extensions.show=../show.py showconfig | grep diff.git
diff.git=True
```
Run the new 'hg show' manually, and confirm that you don't get a git diff despite your hgrc setting:
```
: /data/users/simonfar/fb-hgext (hg) [nocopydetectioninshow]
: simonfar@devvm148 tests $ hg --config alias.show=show --config extensions.show=../show.py show
changeset:   7d57a1e75587af73e252d69611aa12ebffc38e78   (@)
bookmarks:   nocopydetectioninshow
user:        Simon Farnsworth <simonfar@fb.com>
date:        Mon, 11 Jan 2016 16:28:15 -0800

    Turn off git diff by default in
    'hg show'

diff -r 6dda76b19bf1 -r 7d57a1e75587 show.py
--- a/show.py   Mon Jan 11 15:30:19 2016 -0800
+++ b/show.py   Mon Jan 11 16:28:15 2016 -0800
@@ -93,16 +93,27 @@

     opts['patch'] = True
     opts['verbose'] = True
-    backup = ui.backupconfig('ui', 'verbose')
-    basebackup = repo.baseui.backupconfig('ui', 'verbose')
+
+    verbosebackup = ui.backupconfig('ui', 'verbose')
+    verbosebasebackup = repo.baseui.backupconfig('ui', 'verbose')
+    # Copy tracking is slow when doing a git diff. Override hgrc, and rely on
+    # opts getting us a git diff if it's been requested. Ideally, we'd find and
+    # fix the slowness in copy tracking, but this works for now.
+    # See Bryan O'Sullivan's comments on task 9599994 for context
+    gitdiffbackup = ui.backupconfig('diff', 'git')
+    gitdiffbasebackup = ui.backupconfig('diff', 'git')

     try:
         ui.setconfig('ui', 'verbose', True)
+        ui.setconfig('diff', 'git', False)
         # Propagate to subrepos
         repo.baseui.setconfig('ui', 'verbose', True)
+        repo.baseui.setconfig('diff', 'git', False)
         newargs = []
         commands.log(ui, repo, *newargs, **opts)

     finally:
-        ui.restoreconfig(backup)
-        repo.baseui.restoreconfig(basebackup)
+        ui.restoreconfig(gitdiffbackup)
+        repo.baseui.restoreconfig(gitdiffbasebackup)
+        ui.restoreconfig(verbosebackup)
+        repo.baseui.restoreconfig(verbosebasebackup)
```
Run the new 'hg show' manually with --git, and confirm that you get a git diff:
```
: /data/users/simonfar/fb-hgext (hg) [nocopydetectioninshow]
: simonfar@devvm148 tests $ hg --config alias.show=show --config extensions.show=../show.py show --git
changeset:   7d57a1e75587af73e252d69611aa12ebffc38e78   (@)
bookmarks:   nocopydetectioninshow
user:        Simon Farnsworth <simonfar@fb.com>
date:        Mon, 11 Jan 2016 16:28:15 -0800

    Turn off git diff by default in 'hg show'

diff --git a/show.py b/show.py
--- a/show.py
+++ b/show.py
@@ -93,16 +93,27 @@

     opts['patch'] = True
     opts['verbose'] = True
-    backup = ui.backupconfig('ui', 'verbose')
-    basebackup = repo.baseui.backupconfig('ui', 'verbose')
+
+    verbosebackup = ui.backupconfig('ui', 'verbose')
+    verbosebasebackup = repo.baseui.backupconfig('ui', 'verbose')
+    # Copy tracking is slow when doing a git diff. Override hgrc, and rely on
+    # opts getting us a git diff if it's been requested. Ideally, we'd find and
+    # fix the slowness in copy tracking, but this works for now.
+    # See Bryan O'Sullivan's comments on task 9599994 for context
+    gitdiffbackup = ui.backupconfig('diff', 'git')
+    gitdiffbasebackup = ui.backupconfig('diff', 'git')

     try:
         ui.setconfig('ui', 'verbose', True)
+        ui.setconfig('diff', 'git', False)
         # Propagate to subrepos
         repo.baseui.setconfig('ui', 'verbose', True)
+        repo.baseui.setconfig('diff', 'git', False)
         newargs = []
         commands.log(ui, repo, *newargs, **opts)

     finally:
-        ui.restoreconfig(backup)
-        repo.baseui.restoreconfig(basebackup)
+        ui.restoreconfig(gitdiffbackup)
+        repo.baseui.restoreconfig(gitdiffbasebackup)
+        ui.restoreconfig(verbosebackup)
+        repo.baseui.restoreconfig(verbosebasebackup)
```

Reviewers: #sourcecontrol, ttung, lcharignon

Reviewed By: lcharignon

Subscribers: rmcelroy, lcharignon, #sourcecontrol

Differential Revision: https://phabricator.fb.com/D2822071

Tasks: 9599994

Signature: t1:2822071:1452586169:59011b5331cd6c76741cae348c8db4912ed6c9b7

show.py

@@ -69,12 +69,20 @@ def show(ui, repo, *args, **opts):
     opts['patch'] = True
     opts['verbose'] = True
 
-    backup = ui.backupconfig('ui', 'verbose')
+    verbosebackup = ui.backupconfig('ui', 'verbose')
+    # Copy tracking is slow when doing a git diff. Override hgrc, and rely on
+    # opts getting us a git diff if it's been requested. Ideally, we'd find and
+    # fix the slowness in copy tracking, but this works for now.
+    # On a commit with lots of possible copies, Bryan O'Sullivan found that this
+    # reduces "time hg show" from 1.76 seconds to 0.81 seconds.
+    gitdiffbackup = ui.backupconfig('diff', 'git')
 
     try:
         ui.setconfig('ui', 'verbose', True)
+        ui.setconfig('diff', 'git', False)
         newargs = []
         commands.log(ui, repo, *newargs, **opts)
 
     finally:
-        ui.restoreconfig(backup)
+        ui.restoreconfig(gitdiffbackup)
+        ui.restoreconfig(verbosebackup)