diff: index-line: respect --abbrev in object's name

A handful of Git's commands respect `--abbrev' for customizing length
of abbreviation of object names.

For diff-family, Git supports 2 different options for 2 different
purposes, `--full-index' for showing diff-patch object's name in full,
and `--abbrev' to customize the length of object names in diff-raw and
diff-tree header lines, without any options to customise the length of
object names in diff-patch format. When working with diff-patch format,
we only have two options, either full index, or default abbrev length.

Although, that behaviour is documented, it doesn't stop users from
trying to use `--abbrev' with the hope of customising diff-patch's
objects' name's abbreviation.

Let's allow the blob object names shown on the "index" line to be
abbreviated to arbitrary length given via the "--abbrev" option.

To preserve backward compatibility with old script that specify both
`--full-index' and `--abbrev', always show full object id
if `--full-index' is specified.

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -441,10 +441,11 @@ endif::git-format-patch[]
 --abbrev[=<n>]::
 	Instead of showing the full 40-byte hexadecimal object
 	name in diff-raw format output and diff-tree header
-	lines, show only a partial prefix.  This is
+	lines, show only a partial prefix.
-	independent of the `--full-index` option above, which controls
+	In diff-patch output format, `--full-index` takes higher
-	the diff-patch output format.  Non default number of
+	precedence, i.e. if `--full-index` is specified, full blob
-	digits can be specified with `--abbrev=<n>`.
+	names will be shown regardless of `--abbrev`.
+	Non default number of digits can be specified with `--abbrev=<n>`.
 
 -B[<n>][/<m>]::
 --break-rewrites[=[<n>][/<m>]]::

diff.c

@@ -4319,7 +4319,10 @@ static void fill_metainfo(struct strbuf *msg,
 	}
 	if (one && two && !oideq(&one->oid, &two->oid)) {
 		const unsigned hexsz = the_hash_algo->hexsz;
-		int abbrev = o->flags.full_index ? hexsz : DEFAULT_ABBREV;
+		int abbrev = o->abbrev ? o->abbrev : DEFAULT_ABBREV;
+
+		if (o->flags.full_index)
+			abbrev = hexsz;
 
 		if (o->flags.binary) {
 			mmfile_t mf;

t/t4013-diff-various.sh

@@ -239,6 +239,9 @@ diff-tree --root -r --abbrev=4 initial
 :noellipses diff-tree --root -r --abbrev=4 initial
 diff-tree -p initial
 diff-tree --root -p initial
+diff-tree --root -p --abbrev=10 initial
+diff-tree --root -p --full-index initial
+diff-tree --root -p --full-index --abbrev=10 initial
 diff-tree --patch-with-stat initial
 diff-tree --root --patch-with-stat initial
 diff-tree --patch-with-raw initial

t/t4013/diff.diff-tree_--root_-p_--abbrev=10_initial

@@ -0,0 +1,29 @@
+$ git diff-tree --root -p --abbrev=10 initial
+444ac553ac7612cc88969031b02b3767fb8a353a
+diff --git a/dir/sub b/dir/sub
+new file mode 100644
+index 0000000000..35d242ba79
+--- /dev/null
++++ b/dir/sub
+@@ -0,0 +1,2 @@
++A
++B
+diff --git a/file0 b/file0
+new file mode 100644
+index 0000000000..01e79c32a8
+--- /dev/null
++++ b/file0
+@@ -0,0 +1,3 @@
++1
++2
++3
+diff --git a/file2 b/file2
+new file mode 100644
+index 0000000000..01e79c32a8
+--- /dev/null
++++ b/file2
+@@ -0,0 +1,3 @@
++1
++2
++3
+$

t/t4013/diff.diff-tree_--root_-p_--full-index_--abbrev=10_initial

@@ -0,0 +1,29 @@
+$ git diff-tree --root -p --full-index --abbrev=10 initial
+444ac553ac7612cc88969031b02b3767fb8a353a
+diff --git a/dir/sub b/dir/sub
+new file mode 100644
+index 0000000000000000000000000000000000000000..35d242ba79ae89ac695e26b3d4c27a8e6f028f9e
+--- /dev/null
++++ b/dir/sub
+@@ -0,0 +1,2 @@
++A
++B
+diff --git a/file0 b/file0
+new file mode 100644
+index 0000000000000000000000000000000000000000..01e79c32a8c99c557f0757da7cb6d65b3414466d
+--- /dev/null
++++ b/file0
+@@ -0,0 +1,3 @@
++1
++2
++3
+diff --git a/file2 b/file2
+new file mode 100644
+index 0000000000000000000000000000000000000000..01e79c32a8c99c557f0757da7cb6d65b3414466d
+--- /dev/null
++++ b/file2
+@@ -0,0 +1,3 @@
++1
++2
++3
+$

t/t4013/diff.diff-tree_--root_-p_--full-index_initial

@@ -0,0 +1,29 @@
+$ git diff-tree --root -p --full-index initial
+444ac553ac7612cc88969031b02b3767fb8a353a
+diff --git a/dir/sub b/dir/sub
+new file mode 100644
+index 0000000000000000000000000000000000000000..35d242ba79ae89ac695e26b3d4c27a8e6f028f9e
+--- /dev/null
++++ b/dir/sub
+@@ -0,0 +1,2 @@
++A
++B
+diff --git a/file0 b/file0
+new file mode 100644
+index 0000000000000000000000000000000000000000..01e79c32a8c99c557f0757da7cb6d65b3414466d
+--- /dev/null
++++ b/file0
+@@ -0,0 +1,3 @@
++1
++2
++3
+diff --git a/file2 b/file2
+new file mode 100644
+index 0000000000000000000000000000000000000000..01e79c32a8c99c557f0757da7cb6d65b3414466d
+--- /dev/null
++++ b/file2
+@@ -0,0 +1,3 @@
++1
++2
++3
+$