Support exact match on file parts in file: operator

Keep the old "file" field around (using that name in the index for
backwards compatibility), but call it PATH in the code to reflect the
fact that it matches the full path. Add a new "filepart" repeated
field that splits the filename on '/' and adds each unique component
as a field value.

Use "path:" to search only the path, and "file:" to search either the
file or the full path. There is only support for single path
components or the full path, not arbitrary sequences of subcomponents.
The latter is possible but would result in combinatorial blowup of
terms. If users really want to search on that, they can use multiple
"file:" terms (discarding ordering) or use regex search.

Regex search is still over the entire path, but this is implemented at
the ChangeQueryBuilder level and may be changed in the future.

Change-Id: I4f6e06bc3c07989f96e4e74e64113f521c05b9e3

Documentation/user-search.txt

@@ -182,8 +182,8 @@ comment:'TEXT'::
 +
 Changes that match 'TEXT' string in any comment left by a reviewer.
 
-[[file]]
-file:'PATH', f:'PATH'::
+[[path]]
+path:'PATH'::
 +
 Matches any change touching file at 'PATH'. By default exact path
 matching is used, but regular expressions can be enabled by starting
@@ -202,6 +202,18 @@ ones using a bracket expression). For example, to match all XML
 files named like 'name1.xml', 'name2.xml', and 'name3.xml' use
 `file:"^name[1-3].xml"`.
 
+[[file]]
+file:'NAME', f:'NAME'::
++
+Matches any change touching a file containing the path component
+'NAME'.  For example a `file:src` will match changes that modify
+files named `gerrit-server/src/main/java/Foo.java`. Name matching
+is exact match, `file:Foo.java` finds any change touching a file
+named exactly `Foo.java` and does not match `AbstractFoo.java`.
++
+Regular expression matching can be enabled by starting the string
+with `^`. In this mode `file:` is an alias of `path:` (see above).
+
 [[has]]
 has:draft::
 +