fast-import: Support reusing 'from' and brown paper bag fix reset.

It was suggested on the mailing list that being able to use `from`
in any commit to reset the current branch is useful in some types of
importers, such as a darcs importer.

We originally did not permit resetting an existing branch with a
new `from` command during a `commit` command, but this restriction
was only to help debug the hacked up cvs2svn that Jon Smirl was
developing in parallel with git-fast-import.  It is probably more
of a problem to disallow it than to allow it.  So now we permit a
`from` during any `commit`.

While making the changes required to permit multiple `from`
commands on the same branch, I discovered we no longer needed the
last_commit field to be set to 0 during a reset, so that was removed.
(Reset was originally setting the field to 0 to signal cmd_from()
that it was OK to execute on the branch.)

While poking around in this section of fast-import I also realized
the `reset` command was not working as intended if the corresponding
`from` command was omitted (as allowed by the BNF grammar and the
code).  If `from` was omitted we cleared out the tree but we left
the tree SHA-1 and parent commit SHA-1 intact.  This is not what
the user intended in this case.  Instead they would be trying to
reset the branch to have no parent and to have no tree, making the
branch look new-born during the next commit.  We now clear these
SHA-1 values during `reset`, ensuring the branch looks new-born if
`from` does not get supplied.

New test cases for these were also added.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Documentation/git-fast-import.txt

@@ -349,17 +349,16 @@ their syntax.
 
 `from`
 ^^^^^^
-Only valid for the first commit made on this branch by this
-fast-import process.  The `from` command is used to specify the commit
-to initialize this branch from.  This revision will be the first
-ancestor of the new commit.
-
-Omitting the `from` command in the first commit of a new branch will
-cause fast-import to create that commit with no ancestor. This tends to be
-desired only for the initial commit of a project.  Omitting the
-`from` command on existing branches is required, as the current
-commit on that branch is automatically assumed to be the first
-ancestor of the new commit.
+The `from` command is used to specify the commit to initialize
+this branch from.  This revision will be the first ancestor of the
+new commit.
+
+Omitting the `from` command in the first commit of a new branch
+will cause fast-import to create that commit with no ancestor. This
+tends to be desired only for the initial commit of a project.
+Omitting the `from` command on existing branches is usually desired,
+as the current commit on that branch is automatically assumed to
+be the first ancestor of the new commit.
 
 As `LF` is not valid in a Git refname or SHA-1 expression, no
 quoting or escaping syntax is supported within `<committish>`.

fast-import.c

@@ -1667,8 +1667,10 @@ static void cmd_from(struct branch *b)
 	if (strncmp("from ", command_buf.buf, 5))
 		return;
 
-	if (b->last_commit)
-		die("Can't reinitailize branch %s", b->name);
+	if (b->branch_tree.tree) {
+		release_tree_content_recursive(b->branch_tree.tree);
+		b->branch_tree.tree = NULL;
+	}
 
 	from = strchr(command_buf.buf, ' ') + 1;
 	s = lookup_branch(from);
@@ -1936,7 +1938,9 @@ static void cmd_reset_branch(void)
 	sp = strchr(command_buf.buf, ' ') + 1;
 	b = lookup_branch(sp);
 	if (b) {
-		b->last_commit = 0;
+		hashclr(b->sha1);
+		hashclr(b->branch_tree.versions[0].sha1);
+		hashclr(b->branch_tree.versions[1].sha1);
 		if (b->branch_tree.tree) {
 			release_tree_content_recursive(b->branch_tree.tree);
 			b->branch_tree.tree = NULL;

t/t9300-fast-import.sh

@@ -433,4 +433,64 @@ test_expect_success \
 	'sed -e s/pack-.*pack/pack-.pack/ edges.list >actual &&
 	 diff -u expect actual'
 
+###
+### series J
+###
+
+cat >input <<INPUT_END
+commit refs/heads/J
+committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+data <<COMMIT
+create J
+COMMIT
+
+from refs/heads/branch
+
+reset refs/heads/J
+
+commit refs/heads/J
+committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+data <<COMMIT
+initialize J
+COMMIT
+
+INPUT_END
+test_expect_success \
+    'J: reset existing branch creates empty commit' \
+    'git-fast-import <input'
+test_expect_success \
+	'J: branch has 1 commit, empty tree' \
+	'test 1 = `git-rev-list J | wc -l` &&
+	 test 0 = `git ls-tree J | wc -l`'
+
+###
+### series K
+###
+
+cat >input <<INPUT_END
+commit refs/heads/K
+committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+data <<COMMIT
+create K
+COMMIT
+
+from refs/heads/branch
+
+commit refs/heads/K
+committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+data <<COMMIT
+redo K
+COMMIT
+
+from refs/heads/branch^1
+
+INPUT_END
+test_expect_success \
+    'K: reinit branch with from' \
+    'git-fast-import <input'
+test_expect_success \
+    'K: verify K^1 = branch^1' \
+    'test `git-rev-parse --verify branch^1` \
+		= `git-rev-parse --verify K^1`'
+
 test_done