Reland: add a --no-squash argument to git rebase-update

See crbug.com/40264739 for many more details. But a prior attempt
to land this patch resulted in [1] which simply removed all
squashing behavior from `git rebase-update`. That broke several
people's workflows. So this patch attempts, again, to simply
add an opt-in to *not* squashing, for the folks (like me) who
do not want that behavior.

[1] https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6104282

Fixed: 40264739
Change-Id: I4a6aa8e53e854a7e601a0fb83f4f9fcb638d36ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6254099
Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: Mason Freed <masonf@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Auto-Submit: Mason Freed <masonf@chromium.org>

git_rebase_update.py

@@ -140,7 +140,7 @@ def remove_empty_branches(branch_tree):
         print(git.run('branch', '-d', branch))
 
 
-def rebase_branch(branch, parent, start_hash):
+def rebase_branch(branch, parent, start_hash, no_squash):
     logging.debug('considering %s(%s) -> %s(%s) : %s', branch,
                   git.hash_one(branch), parent, git.hash_one(parent),
                   start_hash)
@@ -161,7 +161,8 @@ def rebase_branch(branch, parent, start_hash):
     if git.hash_one(parent) != start_hash:
         # Try a plain rebase first
         print('Rebasing:', branch)
-        consider_squashing = git.get_num_commits(branch) != 1
+        consider_squashing = git.get_num_commits(branch) != 1 and not (
+            no_squash)
         rebase_ret = git.rebase(parent,
                                 start_hash,
                                 branch,
@@ -275,6 +276,11 @@ def main(args=None):
                         '-e',
                         action='store_true',
                         help='Do not automatically delete empty branches.')
+    parser.add_argument(
+        '--no-squash',
+        action='store_true',
+        help='Will not try to squash branches if rebasing fails.')
+
     opts = parser.parse_args(args)
 
     if opts.verbose:  # pragma: no cover
@@ -341,7 +347,8 @@ def main(args=None):
         if git.is_dormant(branch):
             print('Skipping dormant branch', branch)
         else:
-            ret = rebase_branch(branch, parent, merge_base[branch])
+            ret = rebase_branch(branch, parent, merge_base[branch],
+                                opts.no_squash)
             if not ret:
                 retcode = 1
 

man/html/git-rebase-update.html

@@ -756,7 +756,7 @@ git-rebase-update(1) Manual Page
 <h2 id="_synopsis">SYNOPSIS</h2>
 <div class="sectionbody">
 <div class="verseblock">
-<pre class="content"><em>git rebase-update</em> [-v | --verbose] [-n | --no-fetch] [-k | --keep-going]</pre>
+<pre class="content"><em>git rebase-update</em> [-v | --verbose] [-n | --no-fetch] [-k | --keep-going] [--no-squash]</pre>
 <div class="attribution">
 </div></div>
 </div>
@@ -869,6 +869,14 @@ Restoration
 </p>
 </dd>
 <dt class="hdlist1">
+--no-squash
+</dt>
+<dd>
+<p>
+  Skip the squash phase of rebase-update if there are merge conflicts.
+</p>
+</dd>
+<dt class="hdlist1">
 -v
 </dt>
 <dt class="hdlist1">

man/man1/git-rebase-update.1

@@ -32,7 +32,7 @@ git-rebase-update \- Updates all branches to have the latest changes from their
 .SH "SYNOPSIS"
 .sp
 .nf
-\fIgit rebase\-update\fR [\-v | \-\-verbose] [\-n | \-\-no\-fetch] [\-k | \-\-keep\-going]
+\fIgit rebase\-update\fR [\-v | \-\-verbose] [\-n | \-\-no\-fetch] [\-k | \-\-keep\-going] | [\-\-no\-squash]
 .fi
 .sp
 .SH "DESCRIPTION"
@@ -125,6 +125,11 @@ Skip the
 phase of rebase\-update\&.
 .RE
 .PP
+\-\-no\-squash
+.RS 4
+Skip the squashing phase of rebase\-update\&.
+.RE
+.PP
 \-v, \-\-verbose
 .RS 4
 More text than your terminal can handle\&.

man/src/git-rebase-update.txt

@@ -9,7 +9,7 @@ include::_git-rebase-update_desc.helper.txt[]
 SYNOPSIS
 --------
 [verse]
-'git rebase-update' [-v | --verbose] [-n | --no-fetch] [-k | --keep-going]
+'git rebase-update' [-v | --verbose] [-n | --no-fetch] [-k | --keep-going] [--no-squash]
 
 DESCRIPTION
 -----------
@@ -78,6 +78,9 @@ OPTIONS
 --no-fetch::
   Skip the `git fetch` phase of rebase-update.
 
+--no-squash::
+  Skip the squash phase of rebase-update if there are merge conflicts.
+
 -v::
 --verbose::
   More text than your terminal can handle.