ignore: note info/exclude lives in GIT_COMMON_DIR, not GIT_DIR

gitignore(5) says that the per-repository ignore file is
$GIT_DIR/info/exclude, but in a worktree that is not the case:

    git rev-parse --git-path info/exclude
    /path/to/main/worktree/.git/info/exclude
    git rev-parse --git-common-dir
    /path/to/main/worktree/.git

We actually use $GIT_COMMON_DIR/info/exclude. Adjust the documentation
and some code comments to say so.

Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: benknoble

Documentation/git-ls-files.adoc

@@ -331,7 +331,7 @@ can give `--exclude-per-directory=.gitignore`, and then specify:
   1. The file specified by the `core.excludesfile` configuration
      variable, if exists, or the `$XDG_CONFIG_HOME/git/ignore` file.
 
-  2. The `$GIT_DIR/info/exclude` file.
+  2. The `$GIT_COMMON_DIR/info/exclude` file.
 
 via the `--exclude-from=` option.
 

Documentation/git-svn.adoc

@@ -439,7 +439,7 @@ Any other arguments are passed directly to 'git log'
 'show-ignore'::
 	Recursively finds and lists the svn:ignore and svn:global-ignores
 	properties on directories. The output is suitable for appending to
-	the $GIT_DIR/info/exclude file.
+	the $GIT_COMMON_DIR/info/exclude file.
 
 'mkdirs'::
 	Attempts to recreate empty directories that core Git cannot track

Documentation/gitformat-index.adoc

@@ -291,14 +291,14 @@ Git index format
     sequence in variable width encoding. Each string describes the
     environment where the cache can be used.
 
-  - Stat data of $GIT_DIR/info/exclude. See "Index entry" section from
+  - Stat data of $GIT_COMMON_DIR/info/exclude. See "Index entry" section from
     ctime field until "file size".
 
   - Stat data of core.excludesFile
 
   - 32-bit dir_flags (see struct dir_struct)
 
-  - Hash of $GIT_DIR/info/exclude. A null hash means the file
+  - Hash of $GIT_COMMON_DIR/info/exclude. A null hash means the file
     does not exist.
 
   - Hash of core.excludesFile. A null hash means the file does

Documentation/gitignore.adoc

@@ -7,7 +7,7 @@ gitignore - Specifies intentionally untracked files to ignore
 
 SYNOPSIS
 --------
-$XDG_CONFIG_HOME/git/ignore, $GIT_DIR/info/exclude, .gitignore
+$XDG_CONFIG_HOME/git/ignore, $GIT_COMMON_DIR/info/exclude, .gitignore
 
 DESCRIPTION
 -----------
@@ -34,7 +34,7 @@ precedence, the last matching pattern decides the outcome):
    includes such `.gitignore` files in its repository, containing patterns for
    files generated as part of the project build.
 
- * Patterns read from `$GIT_DIR/info/exclude`.
+ * Patterns read from `$GIT_COMMON_DIR/info/exclude`.
 
  * Patterns read from the file specified by the configuration
    variable `core.excludesFile`.
@@ -50,7 +50,7 @@ be used.
    specific to a particular repository but which do not need to be shared
    with other related repositories (e.g., auxiliary files that live inside
    the repository but are specific to one user's workflow) should go into
-   the `$GIT_DIR/info/exclude` file.
+   the `$GIT_COMMON_DIR/info/exclude` file.
 
  * Patterns which a user wants Git to
    ignore in all situations (e.g., backup or temporary files generated by
@@ -97,7 +97,7 @@ PATTERN FORMAT
    match at any level below the `.gitignore` level.
 
  - Patterns read from exclude sources that are outside the working tree,
-   such as $GIT_DIR/info/exclude and core.excludesFile, are treated as if
+   such as $GIT_COMMON_DIR/info/exclude and core.excludesFile, are treated as if
    they are specified at the root of the working tree, i.e. a leading "/"
    in such patterns anchors the match at the root of the repository.
 
@@ -146,8 +146,8 @@ CONFIGURATION
 
 The optional configuration variable `core.excludesFile` indicates a path to a
 file containing patterns of file names to exclude, similar to
-`$GIT_DIR/info/exclude`.  Patterns in the exclude file are used in addition to
-those in `$GIT_DIR/info/exclude`.
+`$GIT_COMMON_DIR/info/exclude`. Patterns in the exclude file are used in
+addition to those in `$GIT_COMMON_DIR/info/exclude`.
 
 NOTES
 -----

dir.c

@@ -2985,7 +2985,7 @@ static struct untracked_cache_dir *validate_untracked_cache(struct dir_struct *d
 		return NULL;
 
 	/*
-	 * We only support $GIT_DIR/info/exclude and core.excludesfile
+	 * We only support $GIT_COMMON_DIR/info/exclude and core.excludesfile
 	 * as the global ignore rule files. Any other additions
 	 * (e.g. from command line) invalidate the cache. This
 	 * condition also catches running setup_standard_excludes()
@@ -3078,7 +3078,7 @@ static struct untracked_cache_dir *validate_untracked_cache(struct dir_struct *d
 		istate->cache_changed |= UNTRACKED_CHANGED;
 	}
 
-	/* Validate $GIT_DIR/info/exclude and core.excludesfile */
+	/* Validate $GIT_COMMON_DIR/info/exclude and core.excludesfile */
 	root = dir->untracked->root;
 	if (!oideq(&dir->internal.ss_info_exclude.oid,
 		   &dir->untracked->ss_info_exclude.oid)) {

dir.h

@@ -153,7 +153,7 @@ struct oid_stat {
  *   - The list of files and directories of the directory in question
  *   - The $GIT_DIR/index
  *   - dir_struct flags
- *   - The content of $GIT_DIR/info/exclude
+ *   - The content of $GIT_COMMON_DIR/info/exclude
  *   - The content of core.excludesfile
  *   - The content (or the lack) of .gitignore of all parent directories
  *     from $GIT_WORK_TREE