[PATCH 2 of 2] templatekw: add a {negrev} keyword

[Jordi Gutiérrez Hermoso]

# HG changeset patch
# User Jordi Gutiérrez Hermoso <jordigh at octave.org>
# Date 1516243120 18000
#      Wed Jan 17 21:38:40 2018 -0500
# Node ID cbf1d676a938e78d40cd3504dd916f787bcb47ee
# Parent  701f8a9defdc09bb63f2596e2fc426f2e78da313
templatekw: add a {negrev} keyword

Revision numbers are getting much maligned for two reasons: they are
too long in large repos and users get confused by their local-only
nature. It just occurred to me that negative revision numbers avoid
both of those problems. Since negative revision numbers change
whenever the repo changes, it's much more obvious that they are a
local-only convenience. Additionally, for the recent commits that we
usually care about the most, negative revision numbers are always near
zero.

This commit adds a negrev templatekw to more easily expose negative
revision numbers. It's not easy to reliably produce this output with
existing keywords due to hidden commits or without a significant
slowdown due to computing a complicated template expression.

diff --git a/mercurial/templatekw.py b/mercurial/templatekw.py
--- a/mercurial/templatekw.py
+++ b/mercurial/templatekw.py
@@ -856,6 +856,12 @@ def showrev(repo, ctx, templ, **args):
 def numrevs(repo):
     return len(repo)
 
+ at templatekeyword('negrev')
+def showrev(repo, ctx, templ, **args):
+    """Integer. The repository-local changeset negative revision number,
+    which counts in the opposite direction."""
+    return scmutil.intrev(ctx) - numrevs(repo)
+
 def showrevslist(name, revs, **args):
     """helper to generate a list of revisions in which a mapped template will
     be evaluated"""
diff --git a/tests/test-obsmarker-template.t b/tests/test-obsmarker-template.t
--- a/tests/test-obsmarker-template.t
+++ b/tests/test-obsmarker-template.t
@@ -2409,6 +2409,23 @@ Check other fatelog implementations
      date:        Thu Jan 01 00:00:00 1970 +0000
      summary:     ROOT
   
+Check that {negrev} shows usable negative revisions despite hidden commits
+
+  $ hg log -G -T "{negrev}\n"
+  @  -3
+  |
+  o  -4
+  
+
+  $ hg log -G -T "{negrev}\n" --hidden
+  x  -1
+  |
+  | x  -2
+  |/
+  | @  -3
+  |/
+  o  -4
+  
 
 Test templates with splitted and pruned commit
 ==============================================
@@ -2559,3 +2576,10 @@ Check other fatelog implementations
      date:        Thu Jan 01 00:00:00 1970 +0000
      summary:     ROOT
   
+  $ hg log -G -T "{negrev}\n"
+  @  -1
+  |
+  o  -2
+  |
+  o  -4
+