revert(3p/git): Revert merge of git upstream at v2.26.2

This causes cgit to serve error pages, which is undesirable.

This reverts commit 5229c9b232, reversing
changes made to f2b211131f.

third_party/git/Documentation/git-commit-tree.txt

@@ -69,6 +69,7 @@ OPTIONS
 	Do not GPG-sign commit, to countermand a `--gpg-sign` option
 	given earlier on the command line.
 
+
 Commit Information
 ------------------
 
@@ -78,6 +79,26 @@ A commit encapsulates:
 - author name, email and date
 - committer name and email and the commit time.
 
+While parent object ids are provided on the command line, author and
+committer information is taken from the following environment variables,
+if set:
+
+	GIT_AUTHOR_NAME
+	GIT_AUTHOR_EMAIL
+	GIT_AUTHOR_DATE
+	GIT_COMMITTER_NAME
+	GIT_COMMITTER_EMAIL
+	GIT_COMMITTER_DATE
+
+(nb "<", ">" and "\n"s are stripped)
+
+In case (some of) these environment variables are not set, the information
+is taken from the configuration items user.name and user.email, or, if not
+present, the environment variable EMAIL, or, if that is not set,
+system user name and the hostname used for outgoing mail (taken
+from `/etc/mailname` and falling back to the fully qualified hostname when
+that file does not exist).
+
 A commit comment is read from stdin. If a changelog
 entry is not provided via "<" redirection, 'git commit-tree' will just wait
 for one to be entered and terminated with ^D.
@@ -96,7 +117,6 @@ FILES
 SEE ALSO
 --------
 linkgit:git-write-tree[1]
-linkgit:git-commit[1]
 
 GIT
 ---