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/diff-generate-patch.txt

@@ -1,12 +1,17 @@
-Generating patches with -p
---------------------------
+Generating patch text with -p
+-----------------------------
 
-When "git-diff-index", "git-diff-tree", or "git-diff-files" are run
-with a `-p` option, "git diff" without the `--raw` option, or
-"git log" with the "-p" option, they
-do not produce the output described above; instead they produce a
-patch file.  You can customize the creation of such patches via the
-`GIT_EXTERNAL_DIFF` and the `GIT_DIFF_OPTS` environment variables.
+Running
+linkgit:git-diff[1],
+linkgit:git-log[1],
+linkgit:git-show[1],
+linkgit:git-diff-index[1],
+linkgit:git-diff-tree[1], or
+linkgit:git-diff-files[1]
+with the `-p` option produces patch text.
+You can customize the creation of patch text via the
+`GIT_EXTERNAL_DIFF` and the `GIT_DIFF_OPTS` environment variables
+(see linkgit:git[1]).
 
 What the -p option produces is slightly different from the traditional
 diff format:
@@ -49,7 +54,7 @@ similarity index value of 100% is thus reserved for two equal
 files, while 100% dissimilarity means that no line from the old
 file made it into the new one.
 +
-The index line includes the SHA-1 checksum before and after the change.
+The index line includes the blob object names before and after the change.
 The <mode> is included if the file mode does not change; otherwise,
 separate lines indicate the old and the new mode.
 
@@ -70,7 +75,7 @@ separate lines indicate the old and the new mode.
       rename to a
 
 
-combined diff format
+Combined diff format
 --------------------
 
 Any diff-generating command can take the `-c` or `--cc` option to
@@ -80,7 +85,7 @@ linkgit:git-show[1]. Note also that you can give the `-m` option to any
 of these commands to force generation of diffs with individual parents
 of a merge.
 
-A 'combined diff' format looks like this:
+A "combined diff" format looks like this:
 
 ------------
 diff --combined describe.c
@@ -113,11 +118,11 @@ index fabadb8,cc95eb0..4866510
 ------------
 
 1.   It is preceded with a "git diff" header, that looks like
-     this (when `-c` option is used):
+     this (when the `-c` option is used):
 
        diff --combined file
 +
-or like this (when `--cc` option is used):
+or like this (when the `--cc` option is used):
 
        diff --cc file
 
@@ -160,7 +165,7 @@ parents.
 4.   Chunk header format is modified to prevent people from
      accidentally feeding it to `patch -p1`. Combined diff format
      was created for review of merge commit changes, and was not
-     meant for apply. The change is similar to the change in the
+     meant to be applied. The change is similar to the change in the
      extended 'index' header:
 
        @@@ <from-file-range> <from-file-range> <to-file-range> @@@