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/technical/api-merge.txt

@@ -28,9 +28,77 @@ and `diff.c` for examples.
 
 * `struct ll_merge_options`
 
-Check ll-merge.h for details.
+This describes the set of options the calling program wants to affect
+the operation of a low-level (single file) merge.  Some options:
+
+`virtual_ancestor`::
+	Behave as though this were part of a merge between common
+	ancestors in a recursive merge.
+	If a helper program is specified by the
+	`[merge "<driver>"] recursive` configuration, it will
+	be used (see linkgit:gitattributes[5]).
+
+`variant`::
+	Resolve local conflicts automatically in favor
+	of one side or the other (as in 'git merge-file'
+	`--ours`/`--theirs`/`--union`).  Can be `0`,
+	`XDL_MERGE_FAVOR_OURS`, `XDL_MERGE_FAVOR_THEIRS`, or
+	`XDL_MERGE_FAVOR_UNION`.
+
+`renormalize`::
+	Resmudge and clean the "base", "theirs" and "ours" files
+	before merging.  Use this when the merge is likely to have
+	overlapped with a change in smudge/clean or end-of-line
+	normalization rules.
 
 Low-level (single file) merge
 -----------------------------
 
-Check ll-merge.h for details.
+`ll_merge`::
+
+	Perform a three-way single-file merge in core.  This is
+	a thin wrapper around `xdl_merge` that takes the path and
+	any merge backend specified in `.gitattributes` or
+	`.git/info/attributes` into account.  Returns 0 for a
+	clean merge.
+
+Calling sequence:
+
+* Prepare a `struct ll_merge_options` to record options.
+  If you have no special requests, skip this and pass `NULL`
+  as the `opts` parameter to use the default options.
+
+* Allocate an mmbuffer_t variable for the result.
+
+* Allocate and fill variables with the file's original content
+  and two modified versions (using `read_mmfile`, for example).
+
+* Call `ll_merge()`.
+
+* Read the merged content from `result_buf.ptr` and `result_buf.size`.
+
+* Release buffers when finished.  A simple
+  `free(ancestor.ptr); free(ours.ptr); free(theirs.ptr);
+  free(result_buf.ptr);` will do.
+
+If the modifications do not merge cleanly, `ll_merge` will return a
+nonzero value and `result_buf` will generally include a description of
+the conflict bracketed by markers such as the traditional `<<<<<<<`
+and `>>>>>>>`.
+
+The `ancestor_label`, `our_label`, and `their_label` parameters are
+used to label the different sides of a conflict if the merge driver
+supports this.
+
+Everything else
+---------------
+
+Talk about <merge-recursive.h> and merge_file():
+
+ - merge_trees() to merge with rename detection
+ - merge_recursive() for ancestor consolidation
+ - try_merge_command() for other strategies
+ - conflict format
+ - merge options
+
+(Daniel, Miklos, Stephan, JC)