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/git-grep.txt

@@ -59,8 +59,8 @@ grep.extendedRegexp::
 	other than 'default'.
 
 grep.threads::
-	Number of grep worker threads to use.  If unset (or set to 0),
-	8 threads are used by default (for now).
+	Number of grep worker threads to use. If unset (or set to 0), Git will
+	use as many threads as the number of logical cores available.
 
 grep.fullName::
 	If set to true, enable `--full-name` option by default.
@@ -93,10 +93,11 @@ OPTIONS
 	with `--no-index`.
 
 --recurse-submodules::
-	Recursively search in each submodule that has been initialized and
+	Recursively search in each submodule that is active and
 	checked out in the repository.  When used in combination with the
 	<tree> option the prefix of all submodule output will be the name of
-	the parent project's <tree> object.
+	the parent project's <tree> object. This option has no effect
+	if `--no-index` is given.
 
 -a::
 --text::
@@ -205,8 +206,10 @@ providing this option will cause it to die.
 
 -z::
 --null::
-	Output \0 instead of the character that normally follows a
-	file name.
+	Use \0 as the delimiter for pathnames in the output, and print
+	them verbatim. Without this option, pathnames with "unusual"
+	characters are quoted as explained for the configuration
+	variable core.quotePath (see linkgit:git-config[1]).
 
 -o::
 --only-matching::
@@ -271,6 +274,23 @@ providing this option will cause it to die.
 
 -f <file>::
 	Read patterns from <file>, one per line.
++
+Passing the pattern via <file> allows for providing a search pattern
+containing a \0.
++
+Not all pattern types support patterns containing \0. Git will error
+out if a given pattern type can't support such a pattern. The
+`--perl-regexp` pattern type when compiled against the PCRE v2 backend
+has the widest support for these types of patterns.
++
+In versions of Git before 2.23.0 patterns containing \0 would be
+silently considered fixed. This was never documented, there were also
+odd and undocumented interactions between e.g. non-ASCII patterns
+containing \0 and `--ignore-case`.
++
+In future versions we may learn to support patterns containing \0 for
+more search backends, until then we'll die when the pattern type in
+question doesn't support them.
 
 -e::
 	The next parameter is the pattern. This option has to be
@@ -330,6 +350,17 @@ EXAMPLES
 `git grep solution -- :^Documentation`::
 	Looks for `solution`, excluding files in `Documentation`.
 
+NOTES ON THREADS
+----------------
+
+The `--threads` option (and the grep.threads configuration) will be ignored when
+`--open-files-in-pager` is used, forcing a single-threaded execution.
+
+When grepping the object store (with `--cached` or giving tree objects), running
+with multiple threads might perform slower than single threaded if `--textconv`
+is given and there're too many text conversions. So if you experience low
+performance in this case, it might be desirable to use `--threads=1`.
+
 GIT
 ---
 Part of the linkgit:git[1] suite