hgrc: introduce HGRCSKIPREPO to skip reading the repository's hgrc

We had a way to change the behavior regarding reading the global and user
config, but we had nothing regarding the repository hgrc itself.

This option is useful in situation where scripts need to be able to work around
strange configuration set by the user in his repository. (and were HGPLAIN is
not enough).

Differential Revision: https://phab.mercurial-scm.org/D7807

mercurial/helptext/environment.txt

@@ -49,6 +49,9 b' HGRCPATH'
     - if it's a directory, all files ending with .rc are added
     - otherwise, the file itself will be added
 
+HGRCSKIPREPO
+    When set, the .hg/hgrc from repositories are not read.
+
 HGPLAIN
     When set, this disables any configuration settings that might
     change Mercurial's default output. This includes encoding,

mercurial/helptext/scripting.txt

@@ -74,6 +74,14 b' HGRCPATH'
     like the username and extensions that may be required to interface
     with a repository.
 
+HGRCSKIPREPO
+    When set, the .hg/hgrc from repositories are not read.
+
+    Note that not reading the repository's configuration can have
+    unintended consequences, as the repository config files can define
+    things like extensions that are required for access to the
+    repository.
+
 Command-line Flags
 ==================
 

mercurial/localrepo.py

@@ -676,6 +676,8 b' def loadhgrc(ui, wdirvfs, hgvfs, require'
     configs are loaded. For example, an extension may wish to pull in
     configs from alternate files or sources.
     """
+    if b'HGRCSKIPREPO' in encoding.environ:
+        return False
     try:
         ui.readconfig(hgvfs.join(b'hgrc'), root=wdirvfs.base)
         return True

tests/test-hgrc.t

@@ -258,3 +258,16 b' source of paths is not mangled'
   plain: True
   read config from: $TESTTMP/hgrc
   $TESTTMP/hgrc:17: paths.foo=$TESTTMP/bar
+
+Test we can skip the user configuration
+
+  $ cat >> .hg/hgrc <<EOF
+  > [paths]
+  > elephant = babar
+  > EOF
+  $ hg path
+  elephant = $TESTTMP/babar
+  foo = $TESTTMP/bar
+  $ HGRCSKIPREPO=1 hg path
+  foo = $TESTTMP/bar
+