merge(3p/git): Merge git subtree at v2.29.2

This also bumps the stable nixpkgs to 20.09 as of 2020-11-21, because
there is some breakage in the git build related to the netrc
credentials helper which someone has taken care of in nixpkgs.

The stable channel is not used for anything other than git, so this
should be fine.

Change-Id: I3575a19dab09e1e9556cf8231d717de9890484fb

third_party/git/Documentation/gitcredentials.txt

@@ -3,7 +3,7 @@ gitcredentials(7)
 
 NAME
 ----
-gitcredentials - providing usernames and passwords to Git
+gitcredentials - Providing usernames and passwords to Git
 
 SYNOPSIS
 --------
@@ -131,7 +131,9 @@ context would not match:
 because the hostnames differ. Nor would it match `foo.example.com`; Git
 compares hostnames exactly, without considering whether two hosts are part of
 the same domain. Likewise, a config entry for `http://example.com` would not
-match: Git compares the protocols exactly.
+match: Git compares the protocols exactly.  However, you may use wildcards in
+the domain name and other pattern matching techniques as with the `http.<url>.*`
+options.
 
 If the "pattern" URL does include a path component, then this too must match
 exactly: the context `https://example.com/bar/baz.git` will match a config
@@ -186,8 +188,110 @@ CUSTOM HELPERS
 --------------
 
 You can write your own custom helpers to interface with any system in
-which you keep credentials. See the documentation for Git's
-link:technical/api-credentials.html[credentials API] for details.
+which you keep credentials.
+
+Credential helpers are programs executed by Git to fetch or save
+credentials from and to long-term storage (where "long-term" is simply
+longer than a single Git process; e.g., credentials may be stored
+in-memory for a few minutes, or indefinitely on disk).
+
+Each helper is specified by a single string in the configuration
+variable `credential.helper` (and others, see linkgit:git-config[1]).
+The string is transformed by Git into a command to be executed using
+these rules:
+
+  1. If the helper string begins with "!", it is considered a shell
+     snippet, and everything after the "!" becomes the command.
+
+  2. Otherwise, if the helper string begins with an absolute path, the
+     verbatim helper string becomes the command.
+
+  3. Otherwise, the string "git credential-" is prepended to the helper
+     string, and the result becomes the command.
+
+The resulting command then has an "operation" argument appended to it
+(see below for details), and the result is executed by the shell.
+
+Here are some example specifications:
+
+----------------------------------------------------
+# run "git credential-foo"
+[credential]
+	helper = foo
+
+# same as above, but pass an argument to the helper
+[credential]
+	helper = "foo --bar=baz"
+
+# the arguments are parsed by the shell, so use shell
+# quoting if necessary
+[credential]
+	helper = "foo --bar='whitespace arg'"
+
+# you can also use an absolute path, which will not use the git wrapper
+[credential]
+	helper = "/path/to/my/helper --with-arguments"
+
+# or you can specify your own shell snippet
+[credential "https://example.com"]
+	username = your_user
+	helper = "!f() { test \"$1\" = get && echo \"password=$(cat $HOME/.secret)\"; }; f"
+----------------------------------------------------
+
+Generally speaking, rule (3) above is the simplest for users to specify.
+Authors of credential helpers should make an effort to assist their
+users by naming their program "git-credential-$NAME", and putting it in
+the `$PATH` or `$GIT_EXEC_PATH` during installation, which will allow a
+user to enable it with `git config credential.helper $NAME`.
+
+When a helper is executed, it will have one "operation" argument
+appended to its command line, which is one of:
+
+`get`::
+
+	Return a matching credential, if any exists.
+
+`store`::
+
+	Store the credential, if applicable to the helper.
+
+`erase`::
+
+	Remove a matching credential, if any, from the helper's storage.
+
+The details of the credential will be provided on the helper's stdin
+stream. The exact format is the same as the input/output format of the
+`git credential` plumbing command (see the section `INPUT/OUTPUT
+FORMAT` in linkgit:git-credential[1] for a detailed specification).
+
+For a `get` operation, the helper should produce a list of attributes on
+stdout in the same format (see linkgit:git-credential[1] for common
+attributes). A helper is free to produce a subset, or even no values at
+all if it has nothing useful to provide. Any provided attributes will
+overwrite those already known about by Git's credential subsystem.
+
+While it is possible to override all attributes, well behaving helpers
+should refrain from doing so for any attribute other than username and
+password.
+
+If a helper outputs a `quit` attribute with a value of `true` or `1`,
+no further helpers will be consulted, nor will the user be prompted
+(if no credential has been provided, the operation will then fail).
+
+Similarly, no more helpers will be consulted once both username and
+password had been provided.
+
+For a `store` or `erase` operation, the helper's output is ignored.
+
+If a helper fails to perform the requested operation or needs to notify
+the user of a potential issue, it may write to stderr.
+
+If it does not support the requested operation (e.g., a read-only store),
+it should silently ignore the request.
+
+If a helper receives any other operation, it should silently ignore the
+request. This leaves room for future operations to be added (older
+helpers will just ignore the new requests).
 
 GIT
 ---