doc/fast-import: document feature import-marks-if-exists

fast-import command-line option --import-marks-if-exists was introduced
in commit dded4f1 (fast-import: Introduce --import-marks-if-exists, 2011-01-15)

--import-marks option can be set via a "feature" command in a fast-import
stream and --import-marks-if-exists had support for such specification
from the very beginning too due to some shared codebase. Though the
documentation for this feature wasn't written in dded4f1.

Add the documentation for "feature import-marks-if-exists=<file>". Also add
a minimalistic test for it.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fast-import.txt

@@ -995,10 +995,14 @@ force::
 	(see OPTIONS, above).
 
 import-marks::
+import-marks-if-exists::
 	Like --import-marks except in two respects: first, only one
-	"feature import-marks" command is allowed per stream;
-	second, an --import-marks= command-line option overrides
-	any "feature import-marks" command in the stream.
+	"feature import-marks" or "feature import-marks-if-exists"
+	command is allowed per stream; second, an --import-marks=
+	or --import-marks-if-exists command-line option overrides
+	any of these "feature" commands in the stream; third,
+	"feature import-marks-if-exists" like a corresponding
+	command-line option silently skips a nonexistent file.
 
 cat-blob::
 ls::

t/t9300-fast-import.sh

@@ -1882,6 +1882,53 @@ test_expect_success 'R: --import-marks-if-exists' '
 	test_cmp expect io.marks
 '
 
+test_expect_success 'R: feature import-marks-if-exists' '
+	rm -f io.marks &&
+	>expect &&
+
+	git fast-import --export-marks=io.marks <<-\EOF &&
+	feature import-marks-if-exists=not_io.marks
+	EOF
+	test_cmp expect io.marks &&
+
+	blob=$(echo hi | git hash-object --stdin) &&
+
+	echo ":1 $blob" >io.marks &&
+	echo ":1 $blob" >expect &&
+	echo ":2 $blob" >>expect &&
+
+	git fast-import --export-marks=io.marks <<-\EOF &&
+	feature import-marks-if-exists=io.marks
+	blob
+	mark :2
+	data 3
+	hi
+
+	EOF
+	test_cmp expect io.marks &&
+
+	echo ":3 $blob" >>expect &&
+
+	git fast-import --import-marks=io.marks \
+			--export-marks=io.marks <<-\EOF &&
+	feature import-marks-if-exists=not_io.marks
+	blob
+	mark :3
+	data 3
+	hi
+
+	EOF
+	test_cmp expect io.marks &&
+
+	>expect &&
+
+	git fast-import --import-marks-if-exists=not_io.marks \
+			--export-marks=io.marks <<-\EOF
+	feature import-marks-if-exists=io.marks
+	EOF
+	test_cmp expect io.marks
+'
+
 cat >input << EOF
 feature import-marks=marks.out
 feature export-marks=marks.new